I've looked for them before, and I'll save you the trouble of hunting through every product - those sheets don't exist. Bit of a shame. You gotta build them by hand.
As for the BV thing...I'd say do it on a case by case basis. Standard engine 'Mechs with longer-ranged, energy-based weapons (ERPPCs, cERLLs, etc) do fine underwater, although you have to plan around their "turret" locations. They work quite well as a secondary line of defense versus more mobile SRT-spammers, so that makes me feel like 'Mechs such as the Banshee -3Mr should cost more than half of their BV. Then, of course, there's stuff like the Blackjack -OU. It might lack UMUs, but it's still one of the largest underwater threats you can bring to the table. Max BV for that one; its jump jets might be useless, but Harjel doesn't cost any BV and yet is an invaluable asset to any underwater unit.
If you're willing to get a bit nitty gritty, you can use a 'Mech design program to help you out. It's not that hard; drop the engine to the smallest possible (moving them 1/2), ditch any weapons or systems that don't work underwater, then use that new BV. So that Banshee -3Mr, for example, would cost 1308 BV. This isn't perfect, and completely falls apart if you're using 'Mechs that don't have any guns with any range at all, but it's a decent way of getting slightly more accurate BVs. I've done it before and it works well enough.
