I will probably get flamed for saying this, but there are times that this law DOES make sense. For instance in front of my place on Mark, it is a pretty busy thru-way on the weekends. If someone ventures out 150'+ from shore they are in serious danger of getting run over. Many boats each day cut the corner too tight and even infringe on 150' which makes even being 100-125' out dangerous. This clearly puts people at risk there, if they travel beyond 150' from shore they are goners...I have had boats inside my moorings which are 90' from shore! We don't allow anyone to swim more than 20' past our swim raft.
Of course the enforceability of it (ticketing swimmers for instance) is idiotic but I can think of other unenforceable/stupid laws on the books that are in place.