I think you might need upgrades for the seal(?)'s speed and a fish finder but I know it's unfinished.
I wasn't sure there was an end-point or not but I started running out of fish and I was $200 short of the Ultra Greatrod, so I had to get the Super Rod. Then I reached the dead end again and went to bed.
SPOILERS(?)
I did find a switch early on and I think it raised a gate further right/down but I'm not sure.