I tend to use the AGM only when I am mounting batteries where they are in accessible (cannot check the electrolyte level), interior, since there is less chance of any hydrogen gas production (it can be pro ducted, but is rare), or have to be mounted on their sides, or ends. There is some argument not to mount upside down, because then the valve regulation (release of H) may not occur properly.
The AGM are much more expensive than lead acid, and I don't feel that the slight increase in rate of charge (most of our outboards can not put out enough amperage to take advantage of that effect) or the more rapid "release" of power again is useable in our small boats. As for the increased longevity there is some controversy still.
As for Yamaha not being compatible with AGM--the company apparently has taken that line--and i have been told two different stories. One was that Yamaha has not done testing with AGM's; the other was that some Yamaha alternators put put 15.1 volts (which is indeed a bit high for AGM on a regular basis. (some battery manufacturers suggest that their AGM be equalized, which does involve relitatively short time of higher voltage) SIM who is a Yamaha dealer initially stated that they did not use AGM with the Yamaha engines, but about 2008 changed that stand, and has been using AGM without problems. There are some reports of blow regulators, but I am not convinced that this is any higher with AGM than lead acid. I had a Yamaha 115 and I never saw more than 14.4 volts on a good digital volt meter I had installed. If indeed, you have a Yamaha, which puts out over 15 volts, then avoid AGM.
I believe that Steve Dashew is still using Lead acid Traction batteries in his boats. You do have to remember that traction batteries are much larger than the conventional batteries which we use. (Even the L 16 mentioned in the off the grid system which was linked to are much larger than the batteries used on C Dories. Generally the less (%) you discharge the batteries, the longer they last. Steve feels that he can live with 80% discharge--and 1200 full cycles in the extreme. If you only go 50% discharge, then the traction batteries can last from 3500 to 5000 discharge cycles. For comparison the AGM Life line batteries are rated for slightly under 1000 cycles.
One of the better sources of information on batteries and their use is at:
http://www.lafdacs.org/pdf_files/Batteries.pdf
Any good quality marine type battery, properly cared for will last 4 to 6 years, you might get a year or more out of a set of AGM's, But traction batteries can last as long as 15 to 20 years...if well cared for. However, out little C Dories cannot carry the weight, and don't require the power that is available. I have a friend who is building a large power cat and he has about 3000 lbs of traction batteries aboard--no 110 volt genet--only a 24 volt 200 amp alternator on a separate diesel, plus the engine 200 amp alternators. Inverters run all of his power needs, including air conditioning.
