strange saying

[Josef]

This seems to have a literal meaning:

The only reason the birds follow the boat is because they can get easy fish.

[southrnpride]

In the context of a relationship it means that basically the only reason one partner is with the other is because there is some sort of extrinsic reward involved. IE when someone dates/marries someone for money.

I could be completely wrong because it's not a saying that's used often where I'm from.