they won't sink, they are less dense than the water and bouyancy will cause them to float. You need to attach something to the veggie that is more dense than water. a spoon works perfectly Just put the handle of the spoon through the veggie and it will sink right to the bottom
You can use stainless steel to sink it. I know some people use forks or metal shish kebab sticks to weigh it down. Just make sure it is stainless steel and clean of any soaps.
Ive also heard that boiling them for a few minutes will make them sink... not sure if anyone can back that up as I haven't tried yet (But will soon!). Waiting for my plecos to come!! As for veggies-- cucumbers, zuccinni, potatoes, green beans, def. peas - but take the skin off first!! That's all I can think of for now. Hope this helps!