Just some concepts of what an anthro thresher would look like in motion swimming. I figure the arms would probably be kept outstretched in front of the snout to keep the pectoral/arm fins in proper position to maneuver.
The gills would be located on the chest, not the neck, as they'd be more closely located right over the gill arches/lungs there, and in swimming position they'd get plenty of water flow over them there. Yes I spend entirely too much time thinking about the 'logical' anatomy of a fantasy critter.

The little top sketches also show the flexibility of an anthro version of a thresher using its tail to strike and stun prey.