How do I trigger function when a character's camera touches a part?

Don't really know how to explain it but here's some code that shows you how to do it. It prints when the camera is inside the box and when it goes outside the box or water.

So just call the function you want called on line 24 where it prints "Camera is inside the box"

local cam = workspace.CurrentCamera
local Zones = workspace.Zones:GetChildren()
local player = game.Players.LocalPlayer
local char = player.Character

while wait() do
    for q = 1, #Zones do
        local cameraX = cam.CFrame.X
        local cameraY = cam.CFrame.Y
        local cameraZ = cam.CFrame.Z
        local Hitparts = Zones[q].Hitbox:GetChildren()
        for i = 1, #Hitparts do
            local PartPos = Hitparts[i].Position
            local PartSizeX = Hitparts[i].Size.X
            local PartSizeY = Hitparts[i].Size.Y
            local PartSizeZ = Hitparts[i].Size.Z

            local RootPartX = char.HumanoidRootPart.CFrame.X
            local RootPartY = char.HumanoidRootPart.CFrame.Y
            local RootPartZ = char.HumanoidRootPart.CFrame.Z

            if cameraX > PartPos.X-PartSizeX/2 and cameraX < PartPos.X+PartSizeX/2 and cameraY > PartPos.Y-PartSizeY/2 and cameraY < PartPos.Y+PartSizeY/2 and cameraZ > PartPos.Z-PartSizeZ/2 and cameraZ < PartPos.Z+PartSizeZ/2 then
                if not inside then
                    print("Camera is inside the box")
                    inside = true
                    outside = false
                end
            else
                if not outside then
                    print("Camera is outside the box.")
                    outside = true
                    inside = false
                end
            end
        end
    end
end

I think it'll be better for the sound to play when the player's head goes under the water, unless your game is first person.