I think you might have an issue between the current switch and where it gets power.

Is it an inline fuse (flat or round glass) or one in the dash?

Where is the current livewell pump grounded? Right to the battery or to a grounding block?

Obviously, a wire comes from the pump, to the switch and then to power. Where does that power wire go? To a main pigtail bag to the battery OR, in my case, to a main power bus located under the console?
For me, I have a power bus and a grounding bus under the console. Obviously, it would be a waste to run a ground wire direct from the pump up to that bus so I would think that it attaches to the main ground to the battery. I suspect yours is the same.

So, here is where I would start. Look at that ground. Maybe pull it off, clean it, and check ANYTHING that is next to it. Next, check your wire from the switch to your fuse. You COULD have a bad fuse block. After that, check the wire to the main power bus. You COULD have a hot wire at the bus double feeding power. It might be worth removing the power, cleaning it, and then inserting it in a different location.

Because this happened all of a sudden, my guess is that a wave (or waves) or constant pounding over several trips could have jogged a connection loose OR, chaffed a wire. It sounds like you are getting more power than needed. If you are not concerned with the pump, A good test would be to put in a larger amp fuse and see if it operates without issues. IF it does, you have a double power feed. If it doesn't, well, you still know where to start.

It is a boat, and they take a pounding. Check the connections. You may find it is as simple as that.