It's all natural water, really, but since they are from the sea, I would suggest salt water. The gold coins are more for passage into Ran's Hall upon death at sea (Ran is cited using a net to drown sailors, so she's not exactly the kind of person who you walk up to and say "Hi").
Aegir, however, is much more hospitable and while you don't need alcohol (which I advise against trying to procure before you're of legal drinking age), you can use baked goods, fruit drinks, milk, and some people like to use coffee in place of alcohol. The idea with Aegir is that he hosts parties, so when giving him offerings, make him feel like he's attending one wherever your altar is.
Personally, I find Aegir would like the chocolate coins more, but a safe bet is using them as an offering to both of them. If you want to work with both of them, a good idea is to honor their nine daughters as well. When I was in an indoor pool with windows during a storm, I would create waves and I felt their presence. It was a rather invigorating experience.
Keep in mind that you don't need physical offerings when you work with the deities (though if you have them, use them) as a lot of Jotnar like actions as offerings. Ran isn't mean, but she's also not for everyone to work with. Aegir also does not like it if you insult Ran in any way. I've not worked with her personally, so I can't offer good advice on how to work with her except to direct you to one of her shrines online (the link is for all of them).