There are basically 2 ways in which we fry seafood in Italy: lightly coated in flour only, or dipped in batter. You can also use just semolina, which will make it quite crispy, or gluten free flour. Everybody has their own batter version, I'll share mine with you.
Will add a few pictures as I go along (I don't eat often deep fried food).
Fish
Mussels must be pre-cooked (how would you get them out of their shells otherwise?) and are usually fried with batter, not just flour
Anchovies are usually just fried with flour
If you use prawns with their shells, use just with flour
Clean prawns, calamari rings, little mediterranean cuttlefish, pieces of solid flesh white fish (such as monkfish, or whole little mediterranean sole, whole little mediterranean mullet, etc.) are perfet both with just flour or in batter.
Prawns: remove the black filament from the back (unless using in their shells) and don't fry them for longer than 1 minute (2 if biggish and with their shells)
Anchovies: gut them, wash them but leave the heads on
Little mediterranean sole and mullet: gut it but leave the head. If a little big, fillet it, or, which would be better, open them like a book
The crab legs ("chele di granchio") that we deep fry in Italy aren't usually the legs themselves, but little oblong shapes made of crab meat with a toothpick sticking out at one of the ends. They are fried with the "impanatura" (flour, eggs, breadcrumbs). I'll write a recipe just for them
Vegetables
Aubergines are usually not coated at all, most Italians swear by the need of cutting them (whichever way you need), then sprinkling them lightly with coarse salt and letting them rest in a colander with some weight on top for 30 mins, followed by rinsing them and patting them dry before frying. I personally don't do this as they do retain too much salt for my liking
Courgettes (zucchini) are perfect both with just flour or in batter, usually cut in julienne strips or in long thick slices
Courgette flowers: absolutely wonderful, perfect with both batter or just flour
Carrots, usually cut in julienne strips, can be fried on their own, or either in batter or just flour. You do need to blanch the carrots before for at least 3 mins (depending how thick your julienne strips are)
Artichokes, usually the small ones, cut in quarters (or eights) and cleaned from the fluffy bits, do need to be placed in a bath of lemon and water for a few minutes, before patting them dry - they're fried just coated in flour
Important rules when deep frying
All your pieces must have the same dimensions within the same batch
Use vegetable oil (peanut oil would be better, as it heats up more) rather than olive oil (which doesn't heat up a lot, but rather starts burning quite soon)
Use plenty of oil: your pieces must be swimming, and must have room, so make small batches at a time (keep your oven preheated at 150 degrees C but turned off and keep the cooked pieces there until ready, keeping the oven door closed, clearly)
In order to accomodate the quantity of oil, and ensure that when the pieces bubble up they won't spill, your pan should be both wide and deep
Your oil must be very hot: 160-180 degrees C is ideal (use a kitchen thermometer the first time at least, so you'll know which setting to use on your hob) - if you don't have a thermometer, put in it a little piece of bread or a drop of batter when you think it's ready: it should immediately float to the surface and get bubbles all over
Make sure to use plenty of dry kitchen paper, renew as needed
Any liquid you add to the batter should be very cold
Any batter you make should rest in the fridge for 15-30 mins: the drastic contrast in temperature between the cold batter and the hot oil will give you the best "dry" fried pieces
NEVER add salt to the batter, the fish/vegetable or the flour: doing so would draw moisture from the fish during frying, and that would make your pieces soft and greasy - any salt should be sprinkled to the food as soon as it's ready
My Batter for Seafood
Ingredients:
125gr flour
1 egg
1 egg white
150ml beer (not stout, any lager will do)
1 tsp dry fast action yeast
1 tsp paprika
Method:
Whip the egg white until very stiff
In a separate wide bowl, beat the egg, then add the rest of the ingredients
Add the whipped egg white
Put it into the fridge for 30 mins, then dip the fish in it, shake any excess and deep fry it immediately in small batches