Plant ID Backlog

So, I have a backlog of plants for my daily blog that I am not entirely sure of the species ID.  Would appreciate some help. I need to learn how to ID plants that don't have flowers on them, any  resources you can recommend for that would be great too.  Thanks so much!

Click on photos to enlarge.

1.  Location: Lake Apopoka, in water. Possibly some type of Oxalis, but they don't seem to grow in water.
2. Location: Lake Apopoka, in water. Water lettuce (Pistia stratiotes)
3. Location: Lake Apopoka, in water. Marsh-pennywort (Hydrocotyle umbellata)
4. Location: Blountston, Florida. Yellow flowery thingie?