You can still certainly go onto the beaches - there's a little one over by Navy Pier and the larger Oak St. beach - but, like the previous poster noted, you wouldn't want to go in the water. It'd be freezin'.

You're perfectly welcome to stroll over to Oak Street Beach....or any other beach in Chicago during March or April. Just remember, however, that they're all officially closed and you might encounter a slight problem with snow.

Seriously, other than being prone to extreme swings from time to time. The weather, on balance, in Chicago during March and April averages out to being fairly similar to the UK. Its just that there's often no such thing as an "average" day!

The water will certainly be cold in March/April!