My favorite gloop is a fairing mixture sold by US Composites, www.shopmaninc.com
, and is a mixture of phenolic microspheres and colodial silica. The phenolic spheres are the reddish brown ones that feather much better than glass spheres. They sand quickly. Since you can use any type of unfilled epoxy resins with it, you can control cure time to suit your situation. If you want quick turn around, use one of the TETA based hardners with a 10 minute pot life.
I do not usually use any polyester based fillers on the outside of the boat or underwater. Epoxy has better adhesive properties and no water absorption. But I do admit to using Bondo on those last little tiny places I always seem to find after spraying primer. By that point I just want to get the whole thing over with!