Tal indeed means dew in Hebrew. The h behind the a doesn't make much sense, though. However, with names you get a lot of varieties.
T-A-L, three letters only. This leaves a lot of room to interpretation and speculation. The German word Tal means valley (cf. dell), and this is where Switzerland comes in. I am not sure about the origin of Wilhelm Tell's last name; I suppose it's from some DIET name (people), but it might as well stem from Tal.
I am sure you will find associations of the kind in many languages.
You will have to ask: Where does the family come from? Jewish or Swiss? Other?