I'll get flamed as well then, my old rib just had a normal car battery in it - nothing else is readily available here and the hassle/delay of getting a proper marine battery shipped here as a one-off made it the easy option. I think it's probably what everybody here does, for the same reason.
In the Halmatic launch at work (twin Volvo diesels) we've recently done away with the marine batteries which were fked and fitted 4 x 110AH Land Rover batteries (2 x 24V banks) rather than the 8 x 6V batteries that it had before, same reason, oh that and the small matter that the quote for the correct marine batteries was $2400 instead of £450! At that price difference we can sling them every couple of years as a precaution and still be cheaper.
Marine batteries are probably a bit like aircraft batteries, where the main difference is that they write "aeroplane" on it and add a 0 to the price