What fish recipe is even better chilled or lukewarm?

Thanks for the great suggestions! I ended up poaching a large piece of salmon and some mushrooms in a miso ginger broth, it was great and the broth itself made for a tasty starter. Very easy preparation for a summer party.

Salmon - one ofmy favorite lunches is leftover grilled salmon over sticky rice with cucumbers & avocado, tuna is lovely cold- olive oil poached or grilled, I think meaty fish over flaky fish-- but thats my taste- And of course lobster, shrimp. calamari, mussels, octopus -- but I feel meh about warm/cool temp clams, but not for any real reason

I like the sound of that.

seared Tuna Carpaccio

Lobster salad... You can cook lobster a day ahead and make the salad before dinner.

From my own experience, any light-fleshed fish steamed and served in a sauce with some acid, like a light fresh tomato sauce or muhammara (Persian walnut pomegranate sauce).
Here's one Syrian version of muhammara from cookbook author Joan Nathan
momentmag.com/exclusive/2009
and a Los Angelese from chef Josef Centeno, including ancho chile:
http://recipes.latimes.com/recipe-ancho-pomegranate-sauce/
Last, Mario Batali is always, or at least often, reminding us that Italians often eat their (cooked) foods at room temperature. Here's an example of what he recommends for the Christmas Eve feast of seven fishes:
http://www.epicurious.com/archive/holidays/christmas/batalisevenfishesserving

Caviar is nice. Domestic is available in California.

Sfogi in saor is popular in Venice. It is sole, fried then marinated in cooked onions, raisins, and pine nuts. It is easy to serve because it is prepared ahead of time and improves as it sits a bit. Check to see if Marcella Hazen has a recipe--I think that is where I found mine. But in case you can’t find that, here is the NY Times version: http://cooking.nytimes.com/recipes/2358-pesce-in-saor-venetian-marinated-fish

Smoked trout. Nice in a salad

I agree with poached salmon. Last summer we had a fundraiser for 200 people in the garden in eastern Washington where the temp. during the day was 100 F. We poached the salmon the day before in a restaurant kitchen, refrigerated it and transported it to the event (about .5 miles away) afternoon of. We put plated it in a large walk in cooler and brought it out to serve when time arrived. It was perfect. Also served a cold green bean salad along side.

A lot of seafood is quite enjoyable as a cold dish. Lobster is mentioned above, but crab and shrimp are also frequently served this way.

While salmon is probably the most common fish in the USA served cold, other fish can do equally well. I've done marinaded sardines and anchovies before, sometimes raw, sometimes grilled then marinaded. Best to remove from the fridge at warm up a bit to be closer to room temperature rather than fridge-cold.

Another old warhorse is seared tuna, whose popularity rose in the Seventies. It's less hip these day, more of a throwback preparation. Of course, fully cooked fresh tuna served cold is definitely a possibility, better than the commercial stuff that comes out of a can.

Any ceviche, as cold as possible.

Cold poached salmon, as HalfPint suggested. Also, roast or grilled salmon served room temp is great, e.g., with a salsa verde. (For me, even better than hot.) Of course lobster served cold with a mayonnaise/aioli is delicious, though pricey for a big group. My recipe here for mussels remoulade is done ahead and served cold - though not really a 'main.'

You could also do a classic Venetian fish (sole, flounder, etc.) prepared 'in Saor' - pan fried the day before, in a sweet/sour marinade with onions, raisins and pine nuts, served room temp. Lots of recipes for it online.

I love cold poached salmon with a herb aioli