See comments below, inserted onto your post
It has been said that we RIB users do not need Deep cycle batterys due to the fact that our boats are not running lights pumps etc for extended periods without re-charging like our fenderless friends.
Agree, however be aware that if you leave it long periods between using your RIB you may be better of with a deep cycle battery.
Also that it is better to have a acid free / sealed battery on a RIB so that during rough sea's the acid does not spill out.
Most batteries you find will be lead acid, belive me you dont want to pay for anything else. However you will find that the "Fredom" type from AC Delco use a different construntion and is sealled.
I would say that most sealed or semi sealed lead acids wont have many problems, unless you use you self righting frame a lot
. You can go for gel cells but they dont come cheap. Also the, I think "Red Start" being sold for marine starting are aemi sealed or sealed.
But also it was said that a sealed / maintance free battery will not be able to cope with the output from a outboard. Apparently a car regulator will put out 11/12 volts continuiosly. Where a outboard will a times pump out up tp 18 volts!
Rubbish, car put out about 13.8 to 14.4 volts, 11/12 wont even charge a 12 volt cell. As for 18 volts, maybe the very small unregulated outboards, i.e. 4hp but for bigger outboards they should be fully regulated and similar to the 13.8 to 14.4 voltages. However some fancy charging regulators may go higher to put current back fast but I have never heard of the going upto 18V, maybe 15.6V but I think that it.
Then it was said that all we needed
If i was fitting a battery to a RIB, i would go for the Leasure type cell which still have lots of current to start, have good self discharge and are cheap, but a normal car battery in my option would do fine.
Hope this helps