すみません、まぐろをワサビ抜きで一皿お願いできますか？ — Excuse me, could I get one plate of tuna without wasabi? (Sushi · Restaurant, Formal — to staff, JLPT N4)

Gesture & etiquette

Point to the item on the menu or a picture if available, or make an 'okay' sign with your hand if you just want to indicate a single plate.

Some kaitenzushi (conveyor belt sushi) restaurants have touch panels for ordering; check for that first. Avoid shouting.