not entirely true... I have been told by a reliable source that this isnt always the case. In the past beaches have been closed to swimmers but not to anglers as this is harder to regulate. One example he used is that they cant go around to different boats asking them not to fish there.....
Different concerns here. Swimming closures are usually due to concerns on urban sewage release, biological pollution, ie. nitrogen level that may cause problems with skins and eyes when exposed. Closures of angling of sportfish and shellfish are due to the detection of alarming heavy metal concentration (mercury, lead).
If there is indeed a concern on the safety of seafood consumption from a particular region, DFO has the mandate to implement a closure, which is done in the form of fishery notices (faxed to post offices, tacklestore, boat launches, information kiosks, emailed to websites, etc). Failure to implement a closure would lead to endangering safety of those who consume the harvested fish or shellfish, so I highly doubt DFO would want to be liable to such damages simply because it is "harder to regulate".
As a general rule, crabs, prawns, shrimp will not inhabit polluted water. If an area is devoid of them, then it's best not to even be fishing there.
Not entirely true as FM has pointed out. If the concentration of mercury or lead reaches a level that cannot be tolerated by inhabitants, then of course nothing would be alive in the water. If the water is polluted with a certain amount of mercury or lead, which would eventually end up in the food chain through filter feeders first, then the next trophic level, etc. Bioaccumulation is the concern in consumption of shellfish or fish that are contaminated with heavy metals. Although one crab swims happily with a small amount of mercury, once you ingest the crab, that amount of mercury ends up in your system. Consuming 9 more crabs that live in the same area, you would have 10 times more mercury in your system than the amount found in one of those crabs. This eventually leads to mercury poisoning. These cases were common back in the 70s and 80s. So, a swimming crab does not necessarily mean a clean crab.