Saltwater Assistance + Wetsuit Assistance = Super Assistance?
At the 48 second mark in the video below, Gary Hall Senior says that, "they" (presumably FINA) "outlawed" saltwater world records in 1969 because the increased buoyancy was assistive. I couldn't find any further details on the decision, though.
There aren't many scientific articles on the subject, but this article conducted the time trials in a pool: Wetsuits, Body Density and Swimming Performance (British Medical journal, 1991)
To eliminate the variables of navigation, currents, eddies, etc, I'd imagine most research would be done in a pool. It makes me wonder if wetsuits are actually more assistive than we realise.