Here is an article you may find beneficial:
https://www.mensjournal.com/adventur...ight-capacity/
The capacity is a suggestion and is typically under rated. Some kayaks come with a certificate of buoyancy. I also don't know if the capacity includes the weight of the kayak or not.
Here is my personal experience: my outback was rated 425# I would take a lot of gear with me:
If you figure:
My weight (full kit, clothed as I would be for fishing, including pfd, radio, water shoes): 220
kayak weight (with drive+seat+paddle+cart):105
Live well (container and battery): 15
Water (assume 4 gal seawater at 8.6#/gal @ sealevel: ~35
rods,reels, gaff, pliars: 20#
fishing tackle (jigs, hooks, weights, etc.): 10
ff+battery (assume SLA battery and modern FF): 7#
Dry bag w/ change of clothes+emergency stuff: 7#
water (hydroflask 40oz., full include bottle weight): 3#
fish bag w/ ice (20# bag): 23#
total weight: 445#
Before any fish I am exceeding the weight limit of 425 (theoretically). But, never once did I feel like I was sinking or my waterline was too high on the kayak, etc.
If you take out the weight of the kayak it would put me at about 340 (20 below the recommended capacity) so maybe if I had another 85# of gear or landed 85# worth of fish (one can dream right), perhaps it would have been a more uncomfortable ride, but I speculate that the buoyancy is underrated on most kayaks.
On a side note saltwater is more buoyant that freshwater, but I don't know how much difference it makes for real life purposes, although I would suspect it gives you a slight advantage over the mfg rating.
Take all of this with a grain of salt, and call the manufacturer if you want to get more info about their process.