Weird attempt to index nil value?

It's hard to tell which line your program is erroring on, since the lines you posted don't match up with what their actual lines in game are.

I'm pretty sure it's erroring because in your function that catches the remote event, you're forgetting that the player is the first parameter since it is implicitly passed. Your boat is the second parameter, not the first, but you're naming the player parameter 'Boat'. The player doesn't exist in the replicated storage so the game will error.

Spawnable = 0
--EDIT: this needs to be a cframe. i THINK lua will ignore the rest of the variables leaving --coordinates to be only 153.375 (either that or itll error) 
--so made sure to wrap this in a cframe.new() so it does what you want it to do
Coordinates = CFrame.new(153.375, 71.75, 43)

script.Parent.Touched:Connect(function(hit)
    if hit.Parent:FindFirstChildOfClass("Humanoid") then
    --player has touched the script.Parent
    end
end)

--EDIT: I changed how the function is ran to be :Connect(function() instead of assigning the function to the OnServerInvoke
-- This is where your issue probably is. 
--The first parameter here should be the player that invoked it, not the boat itself since 
--the player is implicitly passed
script.Parent.SpawnReFunction.OnServerInvoke:Connect(function(playerWhoCalledEvent, Boat)
        if Spawnable == 0 then
        local BReplica = game.ReplicatedStorage:FindFirstChild(Boat):Clone()
        BReplica.Name = Boat
        BReplica.Parent = game.Workspace
        BReplica:SetPrimaryPartCFrame(Coordinates)
    end
end

In case that wasn't the issue, I added some waitForChild() functions. The script might be running before the everything else inside the object was loaded. Look at the comments that say TODO: and make sure to do what it says there in case you're still having the error.

boatGui = script.Parent
boatSelector = boatGui:WaitForChild("BoatSelector")
baseFrame = boatGui:WaitForChild("BaseFrame")
exitButton = baseFrame:WaitForChild("ExitButton")

exitButton.MouseButton1Click:Connect(function()
    boatGui.Enabled = false
end)

--TODO: Check if this path exists vvv
boatSelector.MBoat1.MouseButton1Click:Connect(function()
    --TODO: Put your remote function inside the ReplicatedStorage
    --It is currently confusing since you're invoking the server with an object in the server
    --Check if this path exists vvv
    game.Workspace.BSPP1.SpawnReFunction:InvokeServer("Boat")
end)

Spawnable = 0

Coordinates = 153.375, 71.75, 43

script.Parent.Touched:Connect(function(hit)
    if hit.Parent:FindFirstChildOfClass("Humanoid") then

    end
end)

script.Parent.SpawnReFunction.OnServerInvoke = function(Boat)
        if Spawnable == 0 then
        local BReplica = game.ReplicatedStorage:FindFirstChild(Boat):Clone()
        BReplica.Name = Boat
        BReplica.Parent = game.Workspace
        BReplica:SetPrimaryPartCFrame(CFrame.new(Coordinates))
    end
end