I"ve never been on the Zaandam, but have been on the Volendam, which is her sister ship. We always take a B or BB cat. cabin on Verandah deck, but if you want something cheaper, then I would suggest that you go to Main Deck. We find that the cabins on Main or a bit bigger than those on Lower Prom deck. We like to be either mid-ships or near the aft elevator. The choice is really up to you.
If you haven't booked yet, I wouldn't wait. A lot of the cruises to Alaska are already sold out.
No matter which cabin or deck you pick, I am sure you will love Alaska.