I think I posted some pics for you in a prior thread but I'll try and answer some of your question with example from ours. Not quite sure the purpose of the safety switch since a breaker should perform the same function.
From my main breaker box which is the supply, we put in a double pole 100a breaker. Then from that box we ran about 300ft of buried service cable to a 100amp breaker box at the diffuser. This is the box on the side of the diffuser stand. Inside this box we placed a 20amp breaker for the pump. From there we wired to the inside of the box a GFCI outlet. However my pump is only 120v though. Also, note the waterproof outdoor receptacle with the GFCI.
During the process I actually ran wiring from the breaker box at the diffuser to a spot below the pond and placed a 240v hookup for a camper and another 120v power supply for the fun of it. Inside the breaker box at the diffuser there would be a double pole 30amp breaker for the camper hookup and another 20amp breaker for the 120v power supply when I finally hook them up. The breaker box had 6 open slots.
Big thing to keep in mind is that distance will cause voltage drop so make sure you size your wiring appropriately from your supply to the pump. If I'm not mistaken, we chose aluminum service cable because of cost but had to use 4/0 gauge I think to keep the voltage up. But, we also had a 300ft run. If your supply to pump is only 20ft voltage drop should not be an issue.
On the receptacle, make sure you size the wire to fit the receptacle. On the one we used you pushed the wire in then put the screw in so if its a 12ga receptacle and you use 10ga wire it may not fit to wire it.
Short answer: Yes, you can do a 30a fused safety switch and feed a 20amp but why? If your run is 20ft and you have a breaker box at the supply, I would put in a 20amp double pole breaker there and run to a GFCI outlet inside the box...
