I think you are right but you still sometimes have to move them in and out the boat.
I think they use a 'recombination' vent cap in each of the cells that help keep the electrolyte levels correct but because the battery still needs to vent, they often still have to have something to allow that.
And potentially very explosive. Whilst 'I love the smell of hydrogen in the morning' ... I'm also aware of good ventilation and keeping sparks to a minimum. ;-)
Cheers, T i m