Jim is right: the higher voltages will kill gel batteries.
However, the first thing to determine -- especially with the new batteries -- is exactly which type they really are.
There are several general and very familiar types of lead-acid batteries, e.g., flooded, gelled, and AGM
. There are several less familiar types (and yours may be one of these), e.g., Advanced (hybrid) Gels, Spirals, TPPLs, etc.
While these are all lead-acid batteries, their chemistry, materials, and construction are different.
The type of battery is extremely important to know, because the Iota IQ-4 charging voltages are fixed, as is your alternator
unless you change it. They are generally appropriate for FLOODED lead-acid batteries. AGM batteries have a charging profile very close to flooded batteries, so you can use the same charging voltages without fear.
Gelled batteries, however, are much more sensitive to high charging voltages, and must be treated differently.
FWIW, the inventor of "Advanced (hybrid) Gels" says that their battery chemistry is identical to AGMs, so they can be charged as if they were AGMs.
The hot battery you found is a good indication of an internal short. You should toss it. The other is likely to be compromised, too.
When you put in the new batteries, just be absolutely certain that the connections are clean and tight, and monitor
the batteries closely for the first few weeks.
life should you expect on a cruising boat? IF you take really good care of them, you should get, very approximately:
3-5 years for the cheaper flooded batteries (Cosco, Walmart, etc.)
5-7 years for the better flooded batteries (like the Trojan T-105)
8-15 years for the premium flooded batteries (like the Rolls/Surette)
Less than the flooded battery life for AGMs (fewer cycles, shorter life).
7-15 years for gelled batteries.
Spirals and TPPLs....who knows? Probably 4-7 years.