Before the depression (and there had been no recovery here), I did a lot of work on the electrics of performance cruisers (read - retired racing keep boats). They all had bad electrics and little diesels that are really hard to crank. The universal fix was a rolled cell AGM. (Optima or Orbital type) These do have lower self discharge and much better cranking power for years to come.
The ONLY reason I don't have one as the main engine battery in my coach is the little thing called "Abrogation of Bankruptcy Statues" that relieved me of many years of scrimping and saving for retirement only to have it taken away and given to people that made more most years than I did. If I had known that was going to happen, I would have lived like the grasshopper.....
Matt