Topside paint isn't meant for continuous immersion. Over-coating with bottom paint won't change that. Best practice is to treat the top and bottom sides separately. If you have topside paint on your bottom already, I'd aggresively sand it away until it's ~70% or so gone, and very thin where it remains, then follow the bottom paint prep instructions. It's possible your bottom paint (especially a hard two part finish like VC Performance Epoxy) would dissolve/curl any single part finish underneath it.

   Thanks a lot for your counsel.  It's not what my sanding-tired hands wanted to hear, but live and learn...  :)

I've used graphite (West Systems 423) mixed with epoxy for below the water line on all of my CLC boats and have been very happy with the abrasion resistance and repairability over the years.

<a href="https://ibb.co/x1KvP5g"><img src="https://i.ibb.co/k4VnkcD/bottom5.jpg" alt="bottom5" border="0"></a>

Thank you also, CaptinSkully.   That sounds like a great solution, and one that I've not come across,  I've already repainted the dinghy but will surely tuck that away..