Such tension gages are fairly cheap from NAPA and other 'national' auto aftermarket distributors.
If the belt is too tight the profile of the 'vee' will change and cause uneven wear in the belt sheeves.
If the belt is too loose, it will slip.

Most 'boat applications' 90-100 amperes alternator output at the very MAXIMUM loading of a simple 1/2" vee belt; so one can expect such problems if the belt tension isnt set up 'perfect'.
Use only DAYCO "top-cog" or Gates - blue vee belts. The CRAP vee belts being imported not worth the trouble .... too stretchy, too hard, WRONG profiles to fit in SAE profile sheeves and you probably have a mix of metric and SAE profiles with your sheeves.
If you have an external regulator you can easily 'derate' the alternator so that the power requirement of the belt is lessened; very easy to do with a 'smart' external regulator AND you can change 'how strong' and how 'fast' the alternator 'ramps up' to pull its load.

1/2" vee belts are at the 'hairy edge' of their applicability range in 'pulling' a 90-100 amp load.