The rules of the Schneider cup specified that it was for seaplanes / flying boats, and was a speed competition.
Because it became so prestigious, countries put a lot of effort into producing fast waterborne aircraft in order to meet the rules,hence the seaplane racers.
Had the competition been for landplanes they would no doubt have produced landplane racers, which would possibly have been even faster.
Since, as sea planes, they had an awful lot of oggin for a take - off run, the biggest problem with fixed (coarse) pitch propellors was probably that of keeping straight.
As regards the Swordfish, the difference between take off speed and cruising speed was not that great (think large Tiger Moth), so a fixed pitch prop would work well enough.