I take it you meen inside the inside the canopy ?

yacht varnish will last you for many years, and can be used inside and out ,where as emulsion paint is made mainly for inside purposes and if its out in the elements it will still get wet with condensation and morning dew so unless protected it will peel and flake off unless protected by a coat or two of VARNISH , which somewhat defeats the object of using emulsion in the first place, so in answer to your question use varnish , PS make sure your pond is well covered if you cant move your canopy for varnishing.