One of my AI's took on water while we were sailing them. 85% full at takeout and we didn't notice much difference in performance, but we were in some good wind and waves. I think what helps is the two ama.
absolutely, the biggest problem with a waterlogged hull is not sinking, it's stability. a waterlogged hull will roll erratically, the water inside rushing from side to side and fore and aft unpredictably. the amas act as additional buoyancy cambers and increase the stability in all directions.
as to adding floatation, I'm not saying it's a bad idea or that it will hurt anything, just that it is completely unnecessary. there is a tremendous amount of reserve floatation already. Floatation on a TI: 2 amas with 220 lbs of floatation each, enough foam in the boat to float its weight and then some, 10 to 20% of hull volume in which air is trapped, and any cargo stored in the hull displaces water and is generally lighter than water.
as hull is flooded with water, it's weight does not increase. it's inertia and mass increase and floatation decreases. which isn't the same thing. for example, say the TI hull has 1440 lbs of floatation, 40 lbs of foam floatation, and 2 220 lb amas (these are theoretical to show my point but some might be close (18'*2.6'*.75'*.7= 24cu ft= 180gal*8lbs= ~1440 pounds of floatation in the hull where the .7 is a reducer for sections of the that were included in the measurement but not actually on the boat).
that's 1880 lbs of buoyancy.
the load: 200 lbs for the boat and 400 lbs for the crew.
we have 1280 lbs of floatation. now we remove 90% or 1300lbs of the hull's floatation by adding water. it should sink at this point right? 1300 minus 1280 equals 20 negative pounds! nope! we still have the 40lbs of foam floatation plus as more of the boat sinks below the water the less it counts as negative weight. so, 80 lb boat 90% under water probably weighs only 30 pounds and much of our load is underwater now ie, you are sitting in a puddle which reduces your weight too. so 20% of the crew is underwater or 80 lbs.
adding up the new figures:
floatation: 440 amas +40 foam +140 hull =620 lbs floatation
load: 120 rig +30 hull +320 crew= 470 lbs load
50 lbs positive floatation!
if you are carrying cargo, say a camping load: every cubic foot equals 60 lbs of water displaced. not many things normally carried on a camping trip weight more than water does, so it's really hard to justify wasting valuable hull space with reserve floatation when that cargo is almost as effective as foam would be. every 15 liter drybag filled 2/3s full is 20 lbs of floatation. put a 5 lb stove in it and 2 tanks of propane (which is arguably the densest thing normally carried) and it will float or, at least, barely sink. carry a water bag on deck and it counts against you but carry it in a flooded hull and it has no weight and might even float slightly.
deck cargo is the most dangerous thing we can carry because as we sink it will never add to the floatation and never reduces the load.
in practice, the AI and TI can and will flood to the gunnels but both are virtually unsinkable, which provides you with time to self rescue or wait for help.
so, given the choice between deck cargo, and extra floatation bags and foam noodles in the hull or everything packed into the main hull, i will always choose cargo in the hull.
an interesting aside for the scuba divers here: 1 gallon of water is 8 lbs and 1 gallon of lead is 95 lbs. the tanks will float when empty, the gear is relatively light, but if you're sinking, ditch your weight belts!