Don't forget that water - especially fresh water - is not a good conductor. When you earth the mast you have to consider what it's earthed to, it needs to be able to dissipate the current. I sail often in a large shallow lake and drag a chain to the lake bed when a storm comes, (as well as earthing the mast to the iron keel that has a large surface area). There's lots of good info on the Internet.
e.g Lightning Protection on Sailboats