There are differences between 6V batteries and the typical 12V batteries used in marine applications. No one ever discusses these points so I will.
In this photo you can see the blue line representing low mark for electrolyte levels, and this battery needs topping up. The bottom line is the low mark and the top blue line is the full mark. Even at low this battery still has sufficient electrolyte covering the plates to sustain typical heeling angles if properly oriented.
6V deep cycle batteries are taller than typical "marine" 12V batteries. Why? They are sold and used most widely in golf car applications and the added height benefits these batteries in two important ways.
#1 The added height gives more electrolyte depth covering the plates when compared to typical 12V batteries. Golf carts are often driven on hilly terrain, parked at steep angles and this type of movement is closer to what a monohull sailboat undergoes. Only off road jeeps and four wheelers put flooded batteries through the same pains as a golf car or sailboat... This added case height allows for less chance of the plates uncovering when the golf car moves about on hilly terrain.
#2 In deep cycling applications, like golf cars or sailboats, sulfation of flooded batteries is just a fact of life. The bottoms of 6V battery cases are deeper allowing for the collection of more shed lead sulfate. This battery shows very little sulfate in the bottom of the case but I have seen autopsied batteries with a "snow" covering of lead sulfate in the bottom of the case. 12V batteries can be "shorted" internally due to the build up of lead sulfate which eventually can reach the plates and short across them. This is very rare in 6V deep cycle batteries.