I don’t know if there is particularly a no see-um problem, but any tropical coastal area anywhere in the world will have insects. I don’t recall having any big issues when traveling along the coast, but I haven’t spent a lot of time walking on the beach. Wendy has spent time in Bahia — let’s see how she weighs in on this…
I spent two months in Bahia and never noticed an insect problem there. Of course right on the beach you have a nice breeze which keeps most insects at bay, but even in town I never wore insect repellent and I’m one of those people that attracts every biting insect around!
I suspect if you go farther inland you might find no-see-ums in grassy or swampy areas, but I never noticed a problem at all in places like Bahia, San Vicente, or Canoa.