Depends on the pieces you are working with.
If you are cutting panels with a band-saw or table-saw and then trying to butt join your cut edge to a flat face you really need an epoxy or something that can gap fill.
Weld-on #4 can work great if you are bonding a lap joint or are working with perfectly flat surfaces to butt together. You have to apply it a bit liberally and work fast because it will dry quickly. A syringe tip bottle is great for applying, but a brush can work too. Once it's cured the Acrylic panels will break before the joint will fail.
#3 dries too quickly and is a PITA to use.
#16 is a little thicker and doesn't dry as quickly as #3 or #4, but you still can't dawdle. It won't provide a bond stronger than the base material, but I doubt you need that either.
Honestly though, cyanoacrylate (superglue) also bonds acylic very well if you have nice flat surfaces to join. I've used it to laminate sheets of Acrylic together prior to machining (when i was out of #4).
What I don't know, however, is which is best for long term water submersion.