Bottom Prep for paint
I'd first try a 'Red Devil" hand scraper on an area of the hull. Check with your marina 1st as they may want you to 'catch' the paint scrapings and dispose of them. In any case, ablative paint will scrape off reasonably easily without impacting the underlying gel coat. Multiple coats of Epoxy or vinyl bottom paints may require a soda blast by professionals.
When you've got most of the 'color' off, scratch sand lightly with 60 then 100 grit paper on a hand sanding block or the 100 grit carefully on an orbital sander. Select a bottom paint for your local conditions and use the manufacturer's recommended solvent for a wipe down with clean rags. then roll on two coats of the bottom paint.
As an option, you could first use a barrier coat, such as Interlux 2,000, but if you don't have any blisters, you likely don't need the extra protection. If you do have blisters, that's a completely different story and you'd need to open them up, dry them out, fill them flush, sand & then barrier coat with multiple layers of Interlux 2000, then bottom paint.