I prefer an AGM (Advanced Glass Mat). They are much smaller than a flooded cell, and no matter what direction they face the battery can not leak. They do cost a lot more, but it is for a boat. I use a Hawker Odyssey, and there are many other brands available here in the USA like Lifeline, etc.
This is the battery I have Amazon.com: Odyssey PC925 Automotive and LTV Battery: Automotive
With a heavy electrical load of VHF radio equipped with DSC and a Garmin GPS, plus the electric starter, I can't imagine anything smaller in my own boat.
You will want an onboard battery charger from the engine with a voltage regulator not just a rectifier. AGM's require a different type of battery charger on land and I would recommend one of the CTeK battery chargers as many of them work with the AGM's.