I got a small AGM "sports power" battery for our outboard. Deka ETX30, I think. The outboard is a Suzuki DF15A (15-hp, same basic platform as their current 9.9 and 20-hp models).
Started with the outboard manual, found minimum cranking amp recommendation, went from there. AGM to mitigate self-discharge over winters (I could periodically recharge, but have found that not necessary with AGMs).
South River, Chesapeake Bay