If I'm swimming, the blood is being diverted to my limbs.
If I am standing in the water, no one should be able to tell.
If I am laying on my stomach and tanning, it's a non-issue, besides the discomfort of smashing my dick.
If I am laying on my back, everyone can have a show if they want to stare.
If I am standing, same as with on my back.
You should always have your dick in the hanging position when you put a Speedo on. The material is somewhat restrictive, and tends to hold you in place if you were endowed with anything of significance. When you get hard, it will just look bigger than normal.
If you do this, you'll just look like you've got a dick that is more huge than it usually is. If anyone is ashamed of having his dick look big and having people notice that his dick looks big, he needs a shrink (no pun intended).