Can you get an STD from sharing a bathing suit?

Answer 1

The chances of the boy contracting an STI in this way is slim. Most of these microorganisms require a host to survive, and die within seconds to minutes after leaving the body. There are exceptions, such as various strains of hepatitis which can survive on a solid surface such as a porcelain sink for up to two weeks after leaving the host.

There are other variables to consider as well. If the girl wore the suit in a pool or hot tube that was properly sanitized with chlorine or bromine, it is unlikely that any micro-organizm could survive.

If the boy had any open wounds in the area contacted by the garment, any microorganisms that survived would have an easier vector for tranmission.

None of this is intended to spike paranoia. It is highly unlikely that a transmission of and STI could occur in this scenario. If you are still concerned though, you should consult a physician.
Depends on the STD. HIV needs body fluid contact. Genital Crabs may have survived though through the wash.
no
No
Not if it has a good and thorough wash and dry out in the sun

Answer 2

STDs are spread by exchange of fluids and skin to skin contact. If you didn't have those happen while making out, there's no STD risk.