Hey, I got some suggestions to improve the polar bear more, not sure how possible they are, but hopefully you like them and manage to do them,:

Animation:

When a polar bear catches a fish, maybe it could show its attack animation, to make them feel more alive.

Beef them up:

Polar bears are the largest land carnivore, they are huge and powerful. Why wouldn't that be the case in Minecraft too? Unarmed Piglin Brutes do more damage than a polar bear, so I suggest making them stronger, with more hp too.

My suggestion would be to give them maybe 25 or 30 hearts (A Hoglin having 20, and a Piglin Brute having 25), Polar Bears currently have 15 hearts.

They should hit like a truck, again using the Piglin Brute as a reference, armed with an axe they do the following damage: Easy= 3.75 hearts. Medium= 6.5 hearts. Hard= 9.5 hearts. I believe the polar bear should do the same amount of damage, or maybe a bit less, that could be up to your choice, if you want.

Conservation:

Polar Bears are endangered as we all know, but there is no way to breed polar bears. The reason is obvious, being near a baby bear would make the parents mad, but why not use that in a fun way? I believe that we should be able to breed polar bears, and that could reward you with a lot of xp, that way it would be risky to breed them but with high reward, trying to get close enough to get that xp without dying.

Polar Bears should breed with maybe beef or porkchops, since they eat mostly seals but there are no seals in the game and since this datapack makes them get salmon maybe salmon wouldn't be the best choice? You could decide what to give them I guess.

Polar Bear cubs that you bred, when they grow, should probably attack whoever attacks you if you're around them but would not follow you. Maybe they could heal with salmon.

Extras:
Polar Bears should be fast swimmers, that could be fun. Or deadly if you're trying to escape them.

Is it in any way possible to make polar bears eat the same way pandas do? Or having them put food in their mouths like foxes? I understand if it isn't possible with datapacks, just wanted to suggest it in case it is possible.

Hey, this is looking really good. Maybe provide a version where they fish less frequently, say two-three times a day? Definitely following this.

Fantastic!

Yes yes gun

Hey pretty cool! Totally agree with the featureless features. Can't express how much I like this idea for polar bears! Simple enough, makes polar bears so much more than coincidences.

If you would allow me some thoughts: 1) should be simple enough to summon the fish item entity at the polar bear's mouth rather than the centre of its body? Simple ^ ^ ^ relative coordinates, but that would make it all the more immersive!

2) maybe once 1.16 comes out, you could use the open_water predicate to test if the polar bear is in an open water instead of cooped up in a small cubicle, makes it a little more believable when there's more water?

3) instead of a global timer scoreboard event for every polar bear (it's kinda FISHY that they all catch fish at the exact same time?) maybe an individual timer? Or not even a timer, even more dynamic: some other factor that makes them want to catch fish: perhaps a hunger bar that increases faster when it's raining/snowing? or if there's a baby polar bear closeby? or only if falls below a certain health, then players must find a way to trigger that if they want to make farms? idk if you share my feelings on ultra-automated 1x1 mob farms

Anyways, I am SO looking forward to your final datapack compilation! All the best!