I think the simplest interpretation for this proverb is always make your time productive.
Another possible interpretation is, it's better to do something to achieve your dream than just dream about it.
There are possible two symbolism in this proverb.
weaving the net - represents the things that you can do to achieve your goals.
the fish - represents your goals/dreams in life
watching the fish means that you're just fantasizing and not doing something to achieve your goals and realize your dreams.
I hope my interpretation helps :)