Hydrocaps are used to replace the filler caps on flooded-cell lead-acid batteries. They catalytically convert the oxygen and hydrogen back to water
allowing that to drip back into the cells. They are quite tall and must be protected from impact. Greater vertical space is therefore required for their accommodation.
The bad news is that they are potentially dangerous to use on vessels that might roll in a seaway sufficiently so as to contaminate the caps with electrolyte. Should that happen they actually create so much heat that they melt down and could cause a fire.
I have seen several instances of this and recommend against vessels (other than houseboats in a placid environment) using them. They are designed for stationary use. Yes, it has been true that many boaters use them anyway. Just warning!