Mammals swim other news in 10, we are terrible at it, babies have a pretty good buoyancy due to their mass to size ratio and fat percentage they however aren’t good swimmers what so ever and they don’t swim under water without training you can teach infants to swim but it’s not like they do freestyle out of the womb.
Shellfish have pretty much no omega 3 in them, cold water fish do which are hardly easy to catch in shallow waters.
Nuts and seed however are much richer in Omega 3 than most fish.
Again the aquatic ape theory does not stand up to any level of scrutiny pretty much any claim it makes can be disproven or attributed to other much likely factors it is pseudoscience in its purest form.
The truth is that AAH doesn't match the fossil record (or the fact that apes have been observed swimming even in the wild since it was proposed, the fact that in captivity apes develop similar or higher subcutaneous fat deposits to modern humans, and the average ape still has a higher body fat percentage than a human athlete).
Australopithecus afarensis (Lucy) circa 3.5Ma was bipedal, Australopithecus anamensis 4.2 Ma was bipedal as well albeit with primitive features for the upper body, Ardipithecus ramidus which is about 4.5 Ma had both habitual tree and bipedal adaptations (e.g. could stand upright and likely walk pretty well but had opposable big toes for grabbing at branches).
The earliest clear signs of modern bipedalism (as in a walking gait similar to that of modern humans) is around 3.7 Ma and it's not a skeleton but rather the Laetoli Footprints in Tanzania.
For the most part we had direct and "in-direct" ancestors for about 5 million years with varying degree of bipedalism, we likely haven't been eating seafood at for more than 200-300,000 based on remains found at various sites.
Lastly Shellfish which would be the easiest food source for the shallow water Aquatic Ape is one of the most common food allergies with 3-4% of the population suffering from it, and it causes Anaphylaxis in almost every case unlike other allergies which vary in severity. If we count in seafood allergy in general which includes finned fish then about 2.5% of the adult population suffers from it.
It's quite unlikely that a common food source that impacted our evolutionary path so greatly would have a fairly deadly allergy tied to it.
On the other hand humans are not allergic to mammalian meat (unless they are bitten by a few species of a tick and develop and Alpha-gal allergy) at all, and only in extremely rare cases are allergic to poultry.