Lots of things kill batteries. Stu is right: batteries perpetually on float charge can sulfate. But, they sulfate even faster if left in a partially discharged state habitually.
To be kept in tip-top shape, they need to be float-charged at a high enough voltage -- for FLAs 13.4-13.8 VDC -- AND they need to periodically be "super charged" with an absorption-level voltage -- 14.4-14.8 VDC for a short time.
The higher voltage causes bubbling which tends to dislodge loose PbSO4 lead sulfate crystals from the plates, as well as helps to avoid stratification -- the "layering" of different concentrations of electrolyte.
Ambient temperature can be critical.
You're in Canada
and it's winter where in wintertime very cold temperatures occur. You charge your batteries fully, disconnect them for the winter, and come back in the spring. They'll likely be in pretty decent shape. The cold temps tend to slow the self-discharge process way down.
You're in the Caribbean
. Winter or summer...doesn't matter. You fully charge your batteries, disconnect them, and leave the boat for a few months. You come back and find that you've managed to well and truly murder your batteries. Yep, in just a few months new batteries can be destroyed by leaving them in high temperature places with no charge. Ask me how I know, having had my own boat in the Caribbean
for 11 years and having had to leave it for months at a time.
Now, I tend to leave my high quality battery charger (Victron) on 24/7. It's programmed to float at 13.8VDC then 13.2VDC after inactivity, and every other day it kicks up the voltage to 14.8VDC for 30 minutes. I've found this regime works very well with T-105s and with Crown golf-cart batteries.
This year I left my boat in Maine
for the winter, and there was no way I could have left the charger on full time. Nor did I like the option of having the boatyard plug
the boat in every couple of weeks or so. Result: I brought my eight golf-cart batteries home to VA and put them on charge 24/7. Every month or so I kick up the voltage and occasionally equalize them.
Be sure to always use HydroCaps or WaterMiser caps to reduce loss of electrolyte, especially when the batteries are left on charge for long periods of time.
Note that all the above relates to batteries which are in good shape and in decent installations. If by contrast there is a problem, e.g., with a shorted cell, leaving the charger on and unattended for months at a time could lead to a severe problem.....like a battery exploding or a fire destroying your boat!