Duh, there are lots of polyurethane varnishes out there. Since that
seems to be what is on your boat, stick with it. If you want to change
to spar varnish (and why would you?) sand down to bare wood and put on
a minimum of 8 coats, letting them dry and lightly sanding in between.
Personally, I use Honey Teak and love it. A bit of work to put on, but
very durable and only requires a maintenance coat or two of their
clear every year.