I think you should run your table under any rules you want. But let me witter on before I start pitching some changes for you.
Fit for Purpose
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BattleTech's current underwater rules in a nutshell:
- Half of your weapons no longer work.
- You could catastrophically implode at any moment.
- If it makes you feel any better, your mech runs slightly cooler.
Sauce for the goose. Both you and your opponents will suffer the same penalties. If you fight an enemy on an open plain, the unit with ground movement and range will have an advantage. On broken terrain, a unit with jump jets and close-range weapons will do well. In space, victory will probably go to the one who brough LB-X autocannons and SRMs. Battletech is a game about the terrain as much as it is the units. Water is just another terrain, one that confers benefits and drawbacks for the player to utilize.
Woods hexes and elevated hexes and building hexes all provide ways to interact with advantages and drawbacks which force a player to make a choice when interacting with them. Some units will have more choices than others, but that's the paradigm and it's how games work: players make choices between options to maximize their chances of winning.
TacOps introduces a lot of additional types of terrain and weather to increase those options. The entire a la carte design philosophy of the Battletech game/franchise is based on adding more things to your game/campaign to interact with.
Water in Battletech does offer some choices:
-Units with heat sinks in their legs can dissipate more heat and enjoy partial cover in exchange for losing a lot of MP, having no cover, and a chance to fall.
-Units can take total cover unless another unit follows them into the water to have a knife fight on a parallel map with a lot of dice rolls.
-You can push/charge someone into water so that their hit locations without armor are disabled.
-It can be a less-desirable hex when compared to an already undesirable hex (say, Rubble).
-Fielding non-mech aquatic units.
So Water fulfills its basic game role of offering choices. Nominally. After all, having a battlemech fight entirely in depth two water isn't necessarily the point of having those rules. Like how salt is a great seasoning, but it doesn't make an edible main course. Are the water rules bad if they were only ever intended to be an all-but-impassible terrain type? Well, we have submarines and UMUs and MASS and torpedoes that tell us they weren't "only ever" intended for that.
The modern water rules feel like the result of the grinding of "let's have some water on the battlefield" gears, "we're creating a realized universe which has to include boats" gears, and "let's make some aquatic toys" gears.
So, does Catalyst need to overhaul BattleTech's water rules? I don't think the IP's success depends on it.
Would it be nice for players to have a set of water rules which create a more engaging experience when fighting underwater with 'mechs? Yeees.
The Purpose
So what's not fun about using 'mechs in water?
-PSR rolls: Slows down the game and drains MP that are already at a premium.
-Breach rolls: Slows down the game and creates a random element where any damage can suddenly become critical.
-Ranges/weapons: If you duck into water mid-fight, you might already be close to an opponent, but if you're not then you've got to spend several turns closing.
-High movement costs: If every hex costs 4 MP at least, then your movement options drop precipitously. You have fewer choices and the short range of your weapons dictate most of those options will be "waddle into range."
You don't need to change all of these to make water combat more engaging.
Changing breach and PSR rolls would make aquatic combat a slow slugfest between 'mechs' energy weapons.
Changing the PSR rolls and diminished weapons would make water an arena of quick death from low TMMs and frequent breach rolls.
Changing breach rolls and high movement costs would make fighting in water a push-your-luck slugfest of speed versus (limited) firepower.
Changes
So which rules you'd change depends on what you want that combat to look like. Just a few ideas:
PSR Rolls
-Only require PSR rolls for moving from land to water.
-Only make one PSR per turn for being in water.
-Allow a unit to spend additional MP instead of making a PSR for being in water.
Breach Rolls
-Make breach rolls harder, but give larger weapons/damage clusters a bonus that makes them more likely.
-Require a breach roll only for damage clusters above a certain level (say, half a location's armor)
-Instead of multiple breach rolls, make one hit location roll at the end of a turn a unit was hit. If that location matches one that took damage, then the location is breached. (Very bad for the CT)
-Auto-breach for ever so much damage.
-Forget breaches and just make a determining critical hit roll at the end of the turn for each location hit. Instead of flooding whole locations, individual components are flooded.
Ranges/Weapons
-Allow all weapons to fire underwater
-Allow torpedoes as alternate ammunition for LRMs, SRMs, & MMLs.
-Let energy weapons work normally underwater.
-Let ballistics shoot into/from depth 1 water.
-Let ballistics shoot in water with a reduced range for a fail chance.
-Give ballistics aquatic specialty ammunition.
High Movement Costs
-Reduce the movement cost for depth 2+ water by 1 or 2 points. Reducing it by two points would be a lot.
-Reduce elevation change costs for 'mechs in depth 2+ water by half and let them change up to three elevation at a time.
Patch-Fixes
You could also create equipment for working in water, which doesn't revise water rules, but allows units more easily to circumvent their effects.
LaSMs - Land-sea 'Mechs. Like LAMs, LaSMs can reconfigure themselves to operate well in the sea. They transform like LAMs and once they're transformed, the need to make PSRs is removed and MP cost for water hexes is reduced.
Hydro-Attuned Lasers - Lasers that do okay on land and okay in water.
AES for water - AES for your legs that lets you avoid making water PSRs. Or a feature added to existing AES.
VLMs - Vertical launch missiles. They can shoot from water to targets not in water.
Depth Charges - Missiles that can fire from targets not in water to targets in water.
[Unused fortification superlative] Armor/Structure - A type of armor/structure that better resists breaches or diminishes breach effects. Or an added feature to existing armor/structure types.
You could also let me run into water. It's not really related to your thing, but let me slam a 'mech traveling at 110kph into a lake. I can take it!
Caution and Conclusion
The issue is that if you're on a 'normal' battlefield and your 'mechs are optimized for land combat and your enemies are optimized for aquatic combat, then they might find a lake and sit on it while you each dare each other to come into your tiny sub-battlefield where you have the advantage.
Submarines and aquatic units might be at a disadvantage. They have a niche now and letting 'mechs work in water might obsolete them. Counterpoint: I don't care about vehicles and never will.
If you create weapons that fire from water onto land, it could be very annoying. Albeit, a way to irritatingly force an enemy into your favored terrain, which they aren't optimized for and creates a flailing, unfun way for them to win or a flailing, unfun way for them to lose.
Water rules are clunky, clunkier than they should be given how close they are to the core gameplay experience. But they're not too far out of spec. A full revision isn't necessary, but how you change them depends on how you want to change the gameplay underwater. The best way to tell if your changes are working is to playtest, playtest, playtest.
