You should've found two seeds, and plant them in the sand pile in the squares I've placed in the picture (the sand pile aren't there in the picture because I've already done it.) The blue one is the way to the lake, and the one in the green is the way to the shark. After planting them, shoot them with water and then they'll grow into vines that you can climb on.
Viewing post in Dungeon of the Devourer comments
Going back to this picture, on the left is the rooms where you'll find the seeds. I'm assuming you've found the first seed on the top room by accessing the vines. The second seed is at the bottom left (behind or under the character UI). Hopefully it's enough to help find it.
Edit: This might be the last assistance I give for now because at the time I edit this, I'm about to get sleeping. I wish you luck on finding the other seed.