Most topside coating systems if constantly immersed or kept wet will tend to blister. Provided the boat is only put into the water for a few days then a topside coating can be used below the waterline. Note that if you have a trailer boat with any carpet type material used as a padding against any top-side type coating then you may experience blistering in that area if the carpet remains wet for long periods. For trailer boats that are immersed for longer periods there is the question of - do I antifoul or not? A possible answer here is to use an epoxy primer such as Interprotect^® below the waterline and not overcoat it