People who had never left Britain have been known to develop malaria. Several cases have been described in medical literature.

These people all lived near international airports, and it is believed that mosquitoes carrying malaria parasites had been carried from Africa on aircraft, flown off on arrival and bitten their British victims.
What relevance has this for divers Observations of marine biologists provide a clue. They are noticing species of animals and plants in parts of the world where they would not normally expect to find them.
The arrival of these foreign invaders is not always due to the natural drift of populations as a result of climate changes or from ocean currents. Species appear to be picked up from one part of the world and deposited in another. Species of starfish suddenly turning up on reefs thousands of miles from their natural environment provide an example.
As usual the culprit is man. We are transporting the invaders in our ships. It is not just a few species hanging on to the hulls, but larger numbers carried inside.
When a cargo ship travels without a cargo, it needs to take on ballast. Some vessels find it convenient to use seawater for this purpose, and it is pumped out at the destination to make room for the cargo to be loaded.
Along with that water, all the plankton from the original port will be unloaded. Plankton contains immature forms of animals and plants, and these might colonise the new area.
Other things can be transported in this way, including organisms that can cause illness if swallowed by swimmers and divers, or that might sting and envenomate people in the water. These illnesses could go undiagnosed, because the organism is not expected to be present in that part of the world.
Next time your buddy tells you that he saw a box jellyfish in Swanage Harbour, he might just be right!