TouchEnded Fails to Trigger?

this is because of 1. your debounce and 2. the fact that your character's limbs are constantly bouncing around on the part

you can see it here (seizure warning maybe), the part changes color whenever Touched or TouchEnded fires

obviously with all your limbs being animated and jumping up and down, Touched and TouchEnded are going to fire a lot, so that's part of the reason why you might have been taught to use a debounce with Touched

however, with TouchEnded firing so rapidly, it has a chance to be caught in the middle of the debounce when your character actually does leave

removing the debounce would fix the problem with inconsistency, but it would also cause everything to start acting up a bit, so what we can do in addition to that is add an external hitbox that wraps around the entire character, and only change the TouchingSeaFloor value if that hitbox is what touched or stopped touching the sea floor

for the hitbox, you can put a part in StarterCharacterScripts (yes, i said StarterCharacterScripts; although it's called "scripts", you can put anything in there and it'll be cloned into the character when they spawn, so it's a convenient way to not have to clone things manually), and then make it uncollidable, unanchored, transparent, and massless. make sure that it has a unique name too, because you'll need to reference it in your Touched script

for the size, you want the hitbox to be a bit thicker and taller than the entire character; i'd use a size like [3, 8, 3] but you can change it up a bit depending on how tall or how wide your character is

now that you have the hitbox, you can parent a simple script to it that welds it to the character's HumanoidRootPart

local char = script.Parent.Parent
local weld = Instance.new("Weld")
weld.Part0 = char:WaitForChild("HumanoidRootPart")
weld.Part1 = script.Parent
weld.Parent = script.Parent

so now you have a hitbox that follows your character around and can be used to detect when the player touches the sea floor

now you just have to make a few modifications to your Touched script so that it only detects the hitbox and doesn't have a debounce

local CollectionService = game:GetService("CollectionService")
local selectedpart = CollectionService:GetTagged("OceanFloor")

for _, TaggedPart in pairs(selectedpart) do
    local active = false
    TaggedPart.Touched:Connect(function(hit)
        local human = hit.Parent:FindFirstChild("Humanoid")
        if hit.Name == "Hitbox" and human then -- it doesn't have to be named "Hitbox", but just make sure it has a name that won't clash with anything else
            hit.Parent.TouchingSeaFloor.Value = true
        end
    end)

    TaggedPart.TouchEnded:Connect(function(hit)
        local human = hit.Parent:FindFirstChild("Humanoid")
        if hit.Name == "Hitbox" and human then
            hit.Parent.TouchingSeaFloor.Value = false
        end
    end)
end

now that we're only detecting the hitbox, you can see that Touched or TouchEnded only fires when the character actually touches or leaves the part