Strongly related to #8, but I believe it's a separate issue.

Description

From what I can see, seals breed by themselves, without any player interaction.
Since they are passive mobs, I believe they never despawn.
There doesn't appear to be any limit of the amount of seals in a given area.
That means that over time an infinite amount of seals will accumulate.

I managed to take a screenshot where you can see a swarm of seals breeding

and this is not even all of them, there's about 2-3 times as many of them in that lake - they are basically unstoppable now.

Just like #8, this causes massive server lag.

Suggested Solution

I think seals (or any passive mob, really) should only be able to bread through player interaction, let's say by feeding them fish.
This way the population is directly controlled by the player/s and this is also the way vanilla Minecraft deals with issues like this - there is no vanilla mob that breeds by itself, it is always player invoked.

The obvious alternative solution of adding a limit of how many seals there can be (within a certain area) is harder to implement and still has the issue that the limit will always be reached.

edit: I wanted to add that breading gives xp, you can even see it in the screenshot. This is another reason why they shouldn't breed by themselves.

Ohhh, I think I'm mistaken.

It looks like there's a mod on the server that makes animals eat food from the ground, so they're not breeding by themselves but they are eating the fish, so it is the same issue!