Appetizer for 20 at a Beach House

I agree that at the beach unfussy is best - guacamole, the dips mentioned, cheese, olives...if available maybe some thinly sliced soppressata, prosciutto...

Local seafood can also be lovely, and easy...where we go, there's a shop that smokes local bluefish, mussels, scallops, clams, etc. A dab of sour cream/horseradish/lemon (maybe a few capers) on crackers or slices of baguette. Or steam some shrimp, if that's local to where you are, etc. Takes very little work and is delicious and beach-y.

Stuff a Brie cheese with red pesto (do half-cuts in the Brie and fill them with as much pesto as you can) and then wrap it in tin foil and put it in the oven for 20 minutes (or until all is melted and melded). Serve on top of small toasts. Brie + red pesto is one of those cases in which 1+1=3.

I wouldn't do toasts or bruschetta if you are having pizza for the main course. I'll come back if I can think of a suggestion.

All the dips suggested are great. I did the the spinach borani and it was easy and delicious. A simple hummus and baby carrots dish is a good idea too for those who don't eat gluten. I also often get shrimp and cocktail sauce for these large gatherings.
I wouldn't do anything too fussy with this many people. Since it is casual, maybe a simple salsa, guacamole and chips, which appeals to kids and grown-ups alike. You could try out the genius recipe for guacamole that was featured recently.

For the 2nd app you might want something that can be passed on a (paper!) plate... doesn't require dipping around a central bowl. You're on vacation and that's a lot of people - make it easy. As someone mentioned above, a bruschetta. Slice baguette thinly, toast lightly on a baking sheet in the oven. (I'm assuming you won't have time to rub garlic cloves on the toasts because you'll be too busy getting sand out of your bathing suit). A schmear of goat cheese, thin layer of prosciutto (or not), spoonful of a good tomato salsa spread out on top.

Raw shrimp baked with lime only, mixed with fresh cucumbers, some onion slices , green sauce made of Serrano chilly
This is what we call aguachile is super fresh, easy and delicious, plus it doesn't have tons of calories or fat

Spicy shrimp cocktail.

a nice bruschetta is always refreshing on grilled bread.

Bacon-wrapped dates. Roast in oven. Drizzle with a little balsamic reduction. Very easy and for lack of a better word, yummy.

This is super easy and scales well: http://food52.com/recipes/3541_yoghurt_spinach_dip_borani_esfanaaj_in_the_persian_manner (We were 17 in our beach house; served it with pita chips)

Also, just my opinion, it's a beach house vacation! Put out some nuts and olives, good cheese and crackers, and call it an appetizer!

I always serve Midge's Okonomiyaki as an app. Also - Chef June's Crab Beignets. The newest Genius guacamole with various chips would be super easy.

Smoked trout with horseradish sour cream and fig jam on toast/crackers are always good. Bacon-wrapped, cheese-stuffed dates are great too- the bacon can be halfway cooked ahead of time and the rest finished on the grill. I love this spread: http://www.epicurious.com/recipes/food/views/Fig-and-Walnut-Tapenade-with-Goat-Cheese-105649 as well as these ribs: http://www.epicurious.com/recipes/food/views/Chinese-Bistro-Ribs-109428