[belorn]

> I assume that ocean also has a relatively low density of fish altogether

Interesting assumption but false. The places where fish are located is often pretty dense in fish. That is to say, fish are not evenly distributed. They tend to swim in school of fish, and oceans have vast "desert" areas where the bottom is made of sand and very little else.

The Pacific Garbage Patch is interesting target for cleaning because it has a higher density of plastic compared to other areas. The question is if the density is high. Fishing technique has very little insight to give here beyond technology such as radar and echo sounding, but I am uncertain how effective that would be.

Creating an effective device to clean large swathes of low density ocean is going to require novel designs. For high density areas there are divers, and recreational divers are actually one of the current biggest force in cleaning up water around beaches. The problem is that it does not scale.