I assume you are in a warmer climate and if so, you are ok to leave it in the water.
Ralph hit upon it. I would apply barrier coat but if the boat has been in the water for any length of time, there will be moisture in the hull and you will have to leave it out for several weeks to a month. Many will disagree but the number of hunter water ballast boats I painted personally over the years tells me I know something here. Then apply bottom paint but you will have to either rough up the bottom of the barrier coat to apply the new paint and ask locally what works well in your area.
In addition, add a little chorox to the water ballast tank to keep micro organisms from growing or upon opening the drain bail plug for air to escape or enter the ballast chamber, you will kick yourself with the perfume odor coming from the tank.