I don't know how well a modern drysuit would perform, but I'm sceptical about its ability to do the job. However, in the 1950s and early 1960s, it was considered perfectly in order to use a drysuit when snorkelling or spearfishing in cold waters such as the Pacific Northwest. Back then, drysuits came without the ironmongery on today's suits and the air inside was released either with a simple vent tube at the front or by stretching open the seals at the wrists or the face.
Have a look at the postings I made to the Spearfishing History thread at
https://forums.deeperblue.com/threads/spearfishing-history.106109/, which contain images and information about three period drysuits, one Italian, another American and another Russian. The Russian, or more precisely Estonian, "Tegur" drysuit was in use in the 1990s and may still be worn by a few hardy souls who brave the cold waters of Russia for underwater hunting in wintertime:
It's possible to buy such a suit "new in box" via eBay.
When I snorkel in the North Sea, I use a drysuit that is a replica of the American Totes suit of the late 1950s and early 1960s (The publicity image below is from a 1960 Florida magazine):
The modern replica is black in colour and made by Hydroglove of Idaho and it keeps me perfectly warm when I snorkel in the North Sea off the North East coast of England. Like its predecessor, this two-piece suit has no valves, so suit deflation involves letting air out through the face and wrist seals before fully entering the water. You also have to take care to seal the suit correctly by rolling the jacket and trouser skirts tightly together. Thin clothing worn underneath the thin material of the suit supplies additional warmth.
Finally, I emphasise that my own choice of a Hydroglove suit for sea snorkelling is based on my 50 years of snorkelling experience, going back to the days when these drysuits were more commonplace.
I am not going to recommend suits of this type for everyone, because everybody is different. I've also done my homework researching how to use one and finding out what its limitations are as well as its benefits. It's true that these drysuits take up little space when folded up, but they are also fragile and some kind of extra footwear is needed, particularly on rocky shores. Patching the inevitable small punctures is a necessary skill. If you want to know more, just ask.