My opinion is that the only no-no with a mule relay-fishing from town to town on a boat is a forever-boat.

https://youtu.be/6aZw6OlAueU?t=1187

if fishing for gil is the only purpose...which i mean why wouldn't it be. Then you don't need to get 100 anyway, i know for the general purpose yea 100 would be fun but we all know you can just get soles in lower jeuno, or even pay the 500k and get a boat pass and fish in port.

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Caoimhe - 75 nin pld war blm drk brd drg sam thf cor mnk
Still a nub

Oh you don't need to pay the 500k. Nas conceded to allowing you to gather up the items to do the road to aht urhgan quest as part of getting your mule to another city.
If by port you mean whitegate, there's no money there. But to be honest, if you're limiting yourself to city fishing, targeting pterygotus in nashmau is probably your highest gil/hour. The only problem is that's like the single most remote city zone to send your mule to.