If you prefer a glossy finish, you can get epifanes gloss clear varnish from Jamestown boat supply. It's a little quirky as a finish. First it is thick and needs to be thinned by 50%. Use a foam brush to apply very thinly.
A sponges can do the job,absorbs any drop that overfloated.
If you try the oil varieties, linseed or tung and it's not what you want, you can move onto other finishes once the handle has been washed, dried and use a thinner shellac as base and then use urethane or any other varnish.
Shellac will have a problem with boiled water if the handle remain there for some time
A "crack" will be heard.
I am not a pro but i think there is no need for "waterproof" handle,who's going to carry his brush in a pool or in the sea ?
A "water resist" it's much better term.