all different companies make good batts for hydros. Just check the specs.I'd give you some guidelines,but my standards are alot lower than most peoples on here.
You'll here names like trojan, workaholic, exide and optima,but like i said, they all make alot of different batteries.
I use group 27 deep cycle marine batts on my car.Most riders want nothing but 31s. They need around 1000 C.C.A.s and the higher reserve capacity the better.The only real quality difference between batteries is how many charges they can take in a life time, and how much capacity they loose over time.But many companies will replace batts up to so many years no questions asked (like Northstars that Charging Systems International distributes, so you could just change them out for new ones every three years!