How do I change MaxForce in BodyPosition with a script?

First of all, in advance please excuse my OCD, but I'm not sure why you have a for loop that doesn't affect the code. Also it's quite pointless that you put the following:

for i = 0, 1, 1 do
    --stuff
end

The result is the same if you write this:

for i = 0, 2 do
    --stuff
end

Second, I'm not sure if you already know this (if you do, then my bad) but the MaxForce acts as the capping, or rather self-explanatory maximum force of the BodyPosition. Make sure the 'P' (power) value of the BodyPosition is not 0 as this is the value that actually gives the BodyPosition its force.

Try changing your script to something along the lines of this:

game.Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function()
        game.Workspace.SpeedBoat.BoatControl.BodyPosition = Vector3.new(0, 500000, 0)
    end)
end)

It would be helpful if you could tell me if there are any errors that appear in the studio's console.

If there is something that you are still unclear with I hope this wiki page can help you out:

http://wiki.roblox.com/index.php?title=API:Class/BodyPosition