I'd shy away from painting anything below the water line that spends more then a long weekend, occasionally, w any of the low cost, easy DIY methods that dont involve actual paint intended to be used below the waterline on a boat.
And I wouldnt paint just part of it, I'd paint everything below the waterline so there was no boundary edge of new paint meeting original gelcoat. That edge would be very susceptible to lifting and/or letting water get under it and eventually failing.
WOGs DIY low cost paint is a great option for trailered boats, but not intended for slipped boats, glass or tin.
Woodonglass may have some helpful suggestions aboit paint choices. A few members have used single and 2 stage paint intended for marine use, only 1 I can recall off hand is Supermarine, but there seems to be some concern about the product line(s) that have been posted here still being sold and supported by the maker. IIRC, the website no longer shows those products. May still be a viable option, I dont know 1 way or the other.
