If you are trying to score lot of iron from your destinations and in your own production facilities, but don't have enough fish to run them then yeah, it is worth it to trade some of the octopis into fish and use them to do iron.

My level of iron production in town cost me 13 700 fish and produces 1980 iron. So getting enough fish from opctoi hunt to run the foundry takes 57 crew. So basically 57 crew provides me 1980 iron.

If you would just send those ships to market town to get iron you would only receive 1368 iron.

The forge on the other hand at my level provides exactly the same as what i would receive from market town so not worth it.

So what i try to say is that yes it is worth it to trade for fish to keep your foundry runnin if you otherwise can't by sending few ships to get fish from market town instead of iron.