I was exploring on my horse with full diamond gear and half my inventory full of items that I had to go 2000x2000+ squares from home to get, and one of the times I went into the water, I fell off the horse but was unable to swim. So I drowned and lost everything. Thanks.

Horses are terrible to use, regardless. If a tree is too low to go under (99% of them) then don't let the player go through them. If a tree is 3 blocks high, your head goes into the leaves and the whole screen goes black. If you're in dense forest, you can easily get stuck that way. It's happened to me and I was killed by creepers because of it.

Horses need to be massively fixed. Allow the horses to swim, as well. There is no reason not to have them swim. Horses can absolutely swim, even with someone riding. Why did you make it impossible in this game?

And also I don't know why you can't give some sort of grace period of invincibility (maybe 5 or 10 seconds at most) when going through a nether portal... I have been killed a few times because when I teleported, a ghast or fire slime spawned directly beside me and killed me or knocked me off the platform (since the portal that was generated in the nether on my map is literally floating in the middle of nowhere)

I'm getting very frustrated with small issues like these which have never been fixed in YEARS even though they have obviously been submitted by other people. It's seriously making me not want to play the game, because 80% of the time I go exploring, some bug causes me to die and lose everything. These have all been on survival mode, in a private game and regular generated map. I have no add-ons activated or even installed, so nothing could be interfering with the game mechanics.