A couple of things about this ..
1) that's a LOT of water to heat. The largest snorkel branded hot tub is about 130 cu ft of water, you've got something like 180 cu ft.
2) in addition to the water, you've also got the mass of the cinderblocks and concrete
. You've got a LOT of mass to heat up.
3) the air feed
and exhaust aren't matched. I'm not sure how important this is, but the smaller exhaust creates back pressure which affects the burn quality.
4) Snorkel stoves are indeed almost totally immersed in the water - you've got an air radiator above the surface! So you're losing some heat there.
5) I can't find the link for it, but there is a real issue with inserting the burn chamber into water. Take a step back to consider how an RMH
works... the burn tube mixes oxygen and the gases, and the riser and bell provide a very hot place to ensure all the fuel is converted into heat. The riser tube is insulated to help accelerate the pumping action, and the bell gets very hot (and there is lots of heat remaining to go into a mass). If you've got the body of this in water, it struggles to get hot enough
to do its thing. And really, you don't want it to get that hot in the water - a piece of steel that is hot enough to glow really doesn't get along with water!
So ... we'd like to see a sketch of what you're doing to be sure. I'm worried that most of the heat is just going out the exhaust.
We just had a similar thread
) where some similar issues are discussed - basically I'd like to see an exterior burn chamber and riser, going into a submerged stratification chamber. And the bigger the chamber, the better.