just to pick up on an earlier point in this post
my understanding is that -
1. normal batteries can stand 20% discharge
2. leisure batteries 50%
in both cases though, the battery's life cycle starts to get used in the period between use and recharge (which is why you should always give any new batteries you have lying around a little trickle every month to avoid any deterioration. this is also why small solar panels can be very useful to battery life on boats that aren't used much.
equally deterioration will occur if the above discharge levels are exceeded.
again a generalisation, but leisure batteries usually have more thinner plates, and can weigh more per Ah than normal ones, as well as costing more - so the equations can be quite interesting re which and what capacity for small boats.
for my money I go to the Silver 'normals' which seem to give the best of both worlds - and a 4 year warranty.