If you took that plate out and (semi-permanently - i.e. with stick-on weights) weighted your scooter for fresh water and your heaviest config, then you could have just one thing of weight to add for your lighter batteries, plus the salt water plate to add for salt water, no matter which batteries you are using.
Or, really, you could add stick-on weights to your lighter batteries, to make them weigh the same as your heavier batteries.
Then, you would not have to mess with weight at all, except to add the salt water plate, when you get in salt water.