Recommendations for finishing a workbench
I use Water-based polyurethane. Drys fast and you can get one for floors so it will be a little harder. Put at least three coats on.

You can get a big can and before you use it, divide it into small air-tight plastic bottles, so you always have some around for re-coating the bench if it needs it or other projects. You might consider soothing else than plywood for the top. You can get cheap solid-wood countertops at Home Depot or even Ikea. A thicker top will let you eventually sand the top and re-finish it to make it look new again in 5-10 years. Thicker also = more mass and will feel better it you are hammering etc.