I am using an Ash slab for bathroom vanity top, what is best for sealing and protecting wood from water. Thank you
You have a few options that will provide decent waterproofing, including varnish, lacquer and a one-shot poured finish.
The easiest and probably most economical is polyurethane varnish. It's not the fastest (a pour-on coating would be quickest) but it is simple to apply to a good standard for even the first-timer, by just diluting it somewhat with mineral spirits/white spirit to convert it to "wiping varnish".
Two additional advantages of thinning varnish to wiping consistency are that you don't need to buy brushes or rollers and you won't face the hassle of cleaning them afterward. You will need a supply of clean rags (e.g. scraps of old t-shirts or cotton bedsheets), which must be disposable as you'll let them harden after you're done and then throw them away.
Wiped-on applications of diluted varnish take from a few hours to overnight to harden enough for the next coat, depending on how thickly you apply it, how much you wipe away* and your local temperature and humidity.
I would recommend you apply at least four to six coats, so finishing will take you from a few days to about a week.
*You can wipe away all, some or none of the excess of wiping varnish. It will harden fully regardless of how much you leave on the surface, but takes longer the thicker the film left on the wood. The flip side of this is that is builds a film more quickly too, reducing the number of coats needed.
A little more detail can be found here on Popular Woodworking, Wiping Varnish – What is it? from finishing guru Bob Flexner.