I use a non sealed car battery bought from my local motor factor, which my non rectified old merc requires. The main thing is to keep it topped up with water and to keep it fully charged with a smart charger.
It then seems to last years and years.
A friend recommends a Squadron AGM as being the dogs b.....ks but these are expensive.
I think whatever you get look after it and you should be pleased. I have heard that people buy halfords batterys, stuff them within the 3yrs and take them back and get a fee replacement, of course they never say that they are used/abused on a boat!