BattleTech/MechWarrior jargon.
A "boat" 'Mech is a 'Mech that utilizes only one type of weapon - for example, a 'mech with only lasers.
This is a typical munchkin tactic in MW2 and MW3; It is somewhat harder in MechWarrior 4 because of the hardpoint system (most 'mechs must use different types of weapons). It is legal by rules, but somewhat against the spirit; Many players don't exactly like the thing done by other players, and won't engage in such things themselves, because variety in weapons is often a big advantage...
(Not to be confused with the naval vessels in the games.)