Lagoons in the last few years have had high water and we keep hearing about plans for them to be "modernized", meaning no more habitat for shorebirds.
For now, still good for birding, lots of waterfowl and at the right time of year, shorebirds (May through October, especially good in August/September!). Also, lots of passerines around the edges in the forested areas and bush.

Shorebirds (May-October, but Aug/Sep especially good) & Waterfowl in early Spring and late Summer into Fall. There's always a nice concentration of Bonaparte's Gulls in mid to late August.
Has hosted American Avocet, Western Sandpiper, Hudsonian Godwit and Little Gull amongst others. Great spot for late summer (post-breeding) feeding flocks of swifts and all the swallows in August. 

As with many sewage lagoons in Ontario, they are gated and signed "no trespassing", but there is a general tolerance for birders on the property. Having said that, if workers are present do not enter, and if you are in and workers come in and ask you to leave, please do so with respect. Enter via the eastern end of Cedar Street in Holland Landing.