watery salad

I second the dress immediately before serving comment. However, emulsifying the salad dressing before dressing the leaves (by whisking in a bowl or shaking in a jar) will also help keep the lettuce from wilting (there is debate whether the oil or the vinegar alone is the wilting agent, but Kenji Lopez-Alt thinks it's the oil that breaks down the cellulose). Adding a bit of mustard will help to stabilize the emulsification.

Are you dressing it just before serving? If you added salt or oil in advance, that will cause a salad to become watery. You can usually get away with adding the acid little in advance, although best thing to do is to blend the dressing on the side, and dress the leaves immediately before serving.

(also make sure the leaves are completely dry before dressing them, as Nancy suggested).

Thank you!!

One trick to getting dressing to adhere (and salads not be watery) is to use absolutely dried leaves.
To get them, put them in a terry cloth salad bag or clean cotton pillow case, swing a few times ti shake off residual water, then refrigerate a couple hours in the bag/pillow case. Remove, dress, serve.
If no bag, wrap the greens in a clean cotton dish towel for same process.
Last, I wonder if the manufacturer puts treats the prewashed leaves to keep them fresh and if the chemical interacts with the dressing to make it watery. Try making it yourself one time, starting with normal dirty and/or organic leaves and see if you get better results.