Pipe is specified by its nominal size- 1", 1-1/2" etc and its schedule: 20, 40, 80, etc. The bigger schedule numbers mean heavier walls. Tubing is specified by nominal size and wall thickness.
For towed fish outriggers I would go with pipe, schedule 20 and a large enough diameter to handle the loads. You will need a qualified engineer to figure the loads, specify the pipe and probably reinforce the hull to handle the stresses.
For fishing outriggers, look at other boats and match what they have, probably polished tubing.
David
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