Searching for a good keypad
The membrane keyboard you have linked (first link) in your question should be water resistant, if not water proof.

We use something similar at my work. You can have the keypad stuck to the outside of the enclosure and then have a rubber seal (or silastic) sealing where the ribbon cable enters the enclosure.



You will notice that the ribbon cable pretty much exits the bottom of the keypad. The better ones to get, if you can find them, are the ones where the ribbon cable exits the keypad further away from the edge (see example image below). That way the adhesive backing of the keypad seals the cable entry into the enclosure.



As far as hooking it up to an Arduino - no problems at all. A 16 key matrix keyboard has 8 connections, 4 for each row and 4 for each column. Every time you press a key, the row and column are shorted together.



The way I would connect it up is to have the rows connected as inputs and the columns as outputs. One by one I would turn one of the columns on (HIGH) and see which input(s) are reading a HIGH. If it is I know that that key (row and column) is pressed. This method will handle multiple keys pressed at once.

There is an walk-through tutorial posted at instructables.com.

The more expensive keypad in your second link and this one both have the option of connecting by PS2. There is an Arduino page on connecting and controlling a PS2 keyboard here.