I've never used a macro, tried, but it wasnt efficient.
The easiest is just using the Last Object macro in UO and using Forward/Stop Commands to your ship.
50 is a good start to get familiar with your crabbing, but I found it best to do 150 lobster traps in a straight line.
I lose about 30-35 traps per run doing this method. You just move forward, setting your traps. Then you go all the way back to the beginning and harvest them. Doing this yields 3-5 crabs/lobsters per trap. The great thing is, when you get overloaded, the traps just fall to your feet onboard the ship. Takes about 15 minutes from setting to completing your harvest.
Put a few containers in your hold to empty your crabs into and you'll be rocking em out no problem. Will be a good idea to stock up on 500-1000 traps for a good amount of runs.
Happy Fishing!