Much like the expansions that you can purchase for your farm, you are able to buy expansions for the fishing area too. Initially, various sections will be covered in fog and the only way to unlock them is to save up the required number of Land Deeds, Mallets and Marker Stakes.
Each section that you want to expand into will require a certain combination of resources - you can find out what exactly by tapping on the expansion. There is lots of variety to the areas, from deep and secluded to shallow and open. Depending on the area, you will catch different fish - so it's worth concentrating on expanding into areas unlike too many that you have already.
TIP: You can find more information on where to find certain fish in the Fish sections of this guide.