Layering is good for sure ! I dive in Lake Ontario, so similar conditions. 300 would probably not be enough. I use 400 one piece by BARE, with Fourth Element dry base underneath. My partner uses Four Element Halo, with dry wick underneath. We are both comfortable in 35F-40F waters of the past month for about 40 minutes. I think it is important to have some type of moisture wicking layer next to your skin to take any sweat away from your skin so you don't feel cold and clammy. Oddly enough, you will sweat as your body produces moisture during your activity, and because the suit is a closed environment, you will "sweat" and it will have nowhere to go, so important to get it away from your skin, so you don't get a chill. For these temperature, I like the one piece undergarments as there is no risk of separation for the top and bottom, leaving a nice "exposed" area on your lower back, but that is just my preference. One of my dive buddies use the Fourth Element two piece 400 gram and he is very happy with that. Typically, as we move into May - June, at some point I will switch to 200 gram, but this year may be a little longer for that given the abnormally cool water temps. If you really want to stay warm, you can get the DUI electric undergarment which is awesome, another dive partner wears that and loves it, but it is big $$$$.
Let us know what you settle on and how you like it. Always good to get more perspectives and options for what works.
Happy diving!
