Recommended AGM start battery for a yacht.

Agm batteries not really designed for cranking or short high loads, more suited for house- instruments, lights etc.
Actually the Century Marine Pro you chose would be a good choice. Its a SLA, so essentially won't leak - just don't mount it on its side, or capsize

www.supercheapauto.com.au/p/optima-d31a-optima-yellow-battery/SPO982683.html tv

I have one of these but I nearly always start from the house batteries and the charging of the optima is from the house batteries via VSR. Start can be achieved from any of the 3 batteries I have on the boat. I consider the optima to be an emergency/aux that is always charged and provides grunt.

I use a sealed standard auto lead acid for a 50hp yanma no problem whatsoever. Its been doing its thing for over 4 years.

House batteries are 6 x 6volt USA 125 wetcells wired series parallel. Which replaced the Trojan 120 wetcells. The Trojans were in there for 10 plus years.
They are also used in golf carts & forklifts. Not your traditional deep cycle type. Old school some say.
They will crank the engine and windlass quit easily as well.

They are tough won't catch fire, will suffer quite a beating, won't suffer thermal runaway. Designed to be on a moving platform.
Charging is also straight forward. They will suffer accidental overcharge better than the exotic types.
You do have to check water every month though.

These batteries when upside down will only just drip a bit.
Not good you say.
But as its a sloop should come back up in 5 sec or so. I guess if this happens a few tea spoons of acid in the bilge maybe the least of your worries.

Some people think acid will gush out of wet cells when tilted.
This simply is not the case unless one leaves the caps off!!

There is a hazard of hydrogen gas production during charge but it is minimal and vents out really quick & wont collect in bilges.

Never heard of a hydrogen explosion in any land vehicle really.
Only LPG or ....solvent ....(sorry Magnus)

My Century Marine start battery died after 3 years. I have had to use my 120 amp h AGM deep cycle to start my 35hp engine with no issues. I did some research on using deep cycle agm batteries as a start battery and you can provided you go for a battery that is 20% amp capacity over what the normal start battery would be. I now have a 140 amp h Giant AGM for the house battery and use the Giant 120 AGM amp for the starter. Apparently Giant AGM Batteries can last up to 12 years if only lightly cycled (according to aussiebatteries.com.au). Note my engine is near new and the vessel located in a mild temperate climate. I would not do this in Tasmania with a 30 plus year old diesel in poor condition.

I went down the Aussie battery/ giant agm path for the house battery and it let me down at the two year mark, the 5 year pro rata warranty was just enough to keep me in the giant loop, unless I get outstanding service from the replacement unit, I will be going with better name brands, optima, century etc. for start batteries I've got a pair of interstate z70 wet cells that are 5 yrs in service but their only duty is to start the donkey