I have been using Oddysey AGM batteries for many years now. The draw back is the price is usually a $100 more or so. But they are very light and built to standards way beyond a cheap old wet cell lead battery. And can last three times as long. Sears Platinum is a a Oddysey of a different color. And you get a better warranty than Oddysey offers at more than $50 less. But it's still pricey though
If it were me I'd be looking at the standard Walmart Deep Cycle gp24 /gp27 or Costco Interstate DC gp 24. Should be around $77 + core charge. I have used both with success. I am currently on my 3rd year with the Interstate gp27 on my MinnKota Terrova 55# and its still going strong. I think the key is to recharge properly after each trip and to check the water levels a couple of times each year. Works well for me.
Sounds like the issue is the flush deck mount trapping water. You may be able to just cover the existing hole with a small sloped receptacle housing and solve the issues of keeping the plug dry and the underneath compartment waterproof.
Check out this discussion on 2Coolfishing on the same subject. Two posts have pictures of the housing.
For 100.00 it is hard to beat the Walmart Everstart Max group 29. I'm replacing mine after 4 years of use with 55lb thrust IPilot. I'm convinced the only reason I'm replacing it is because I forgot to recharge it after working it all day then it sat for 2 weeks. Had I recharged it I think it would still be going strong.