RTV

[Rocky]

Any reason not to use it for sealing something below the waterline? I want it to be easy to remove if necessary.

[Ken Hutchins]

Don't use any kind of silicone on anything you may want to paint now or in the future. It will stick to most anything but nothing else will stick to it. Use a good non hardening marine bedding compound.
This caution also applies to silicone based polishes etc. Any silicone residue no matter how small will cause 'fish eyes' in the paint.