what about increasing their torpedo damage? If there was a concern it'd be too powerful their range could be reduced slightly since with their slow speed they'd then function relatively similar to hover tanks in that you can try and attack naval units but the naval units can kite and/or run away.
They have just over twice the health of a sub and slightly less mass cost, but a fraction of the DPS (7.5 vs 37.5). Doubling their dps to 15 would still make them significantly weaker than a sub at naval combat (where the enemy stays in range), but might be enough for them to be usable as a last resort. However I dont know if this would cause any other issues from a naval balance perspective.