But, unless this is a new boat it is unlikely you will sand through the existing bottom paint and into the gel coat. You lightly sand to create "tooth" for better adhere-ing of the new paint. From there it gets more complicated. But if you just want to recover existing bottom paint and you are using the same or similar paint, and its not a ablative paint, then light sanding and a wash are good practice. If you are in a boatyard you'd better check their rules and if they allow you to sand they will want you to collect the dust. Just remember that the guy down wind of you may have just varnished his toe rail when you start to sand. And here on the East End of LI most of the yards do not allow owners to paint their bottom citing pesticide application rules.
Also, I usually touch up the bottom paint rather than repaint every year. I also scrub the bottom often during the season. But I don't want a lot of paint building up on the bottom. I believe the yard uses Interlux Bottomcoat - or at least that's what they charge me for.