Spirulina, the most common form of seaweed algae used in supplements can provide omega-3 fatty acids similar to fish oils. For best results, it should be combined with a nut or seed oil, such as flaxseed oil.
If you are looking for a vegetarian source comparable to fish oil, try flaxseed oil. It has a large concentration of omega-3 fatty acids. For a fish oil, cod liver oil or krill oil would be a good choice.
There are some veg "fish oil" supplements, like Seven Seas Vegetarian Omega-3. But you don't have to limit yourself to that. What you are after is the Omega-3 fatty acid, which can come from other sources like flax seed. You can also get your Omega-3 needs met using currant oil, or algae mixed with nut oils.
Seaweed based fish oil supplements for vegetarians are appropriate for use by vegetarians who eat fish and/or fish oil products and substances derived from fish. Seaweed has long been known for its nutritional properties which contain a high amount of B vitamins and minerals. Fish oil supplements contain nutritional properties such as heart-healthy Omega-3 fatty acids.