What are short swim trunks called?
Originally known as surf trunks, later as Jams, and occasionally in British English as swim shorts, boardshorts are a style of men's and, more recently, women's summer wear. Boardshorts have evolved over the last 50 years and separated themselves from the simpler swim trunks category from which they originated.
Australians might say "swimmies"
In BrE, we just say trunks (for men) if it's clear from the context what you mean and "costume" for women