personally I recommend the use of Nemys (https://nemys.ugent.be/) for identification of nematodes. It has been created by the experts of the University of Gent and it's trustworthy. You can select an option that allows to consider not only marine nematodes, so you'll get freshwater and brakish water nematodes as well. If you already have an idea of the nematode you are looking for, use "search taxonomy" from the menu you have on top of the page. If you create your account, you'll be able to download the relative papers, which is a very useful option.The Handbook of Zoology (https://www.degruyter.com/document/doi/10.1515/9783110274257/html) is also a good tool to put next to Freshwater Nematodes.