Thinning bottom paint is generally not recommended unless you're painting a wooden boat. On bare wood, you would thin the bottom paint 10 to 20% on the first coat, using the manufacturer's recommended thinner/reducer.
In dry hot weather conditions, the bottom paint may require some thinning in the roller tray as the solvent flashes off. Always follow the bottom paint manufacturer's recommended solvent for thinning, and usually only thin up to 10%.
Bottom paint contains heavy solids like cuprous oxide that will settle to the bottom of the can and be very thick and hard to mix. It's best to use a mechanical shaker, if available, or you can use a drill mixer. Be sure to mix the paint thoroughly, to a uniform color and viscosity, before adding any thinner, or before use.