Peas do not like to transplant. I would still try to stick them in at the end of may in a cooler spot if you have one. Here I grow them all summer and they are fine. I put my garlic in last fall as well. And generally, I have heard, that root crops are things that you generally don't want to transplant as it disrupts the part of the plant that you really want to grow. Although, I guess onions would be an exception that that. So I guess I don't know! Watermelon I would definitely start. Are you growing anything else. Greens transplant well, squash, herbs, and tomatoes are all things that come to mind.