In the SCUBA showcase, I was forever being detected by enemies whenever I attempted to sabotage the boat; baring in mind that when I did so, I was underneath the boat where I could not be seen. I was also detected whenever I 'surfaced' the water, regardless of how far away I was or how choppy the waves were.
On the other hand, in the driving showcase, the enemy failed to detect me when I stood up from behind cover approximately 5m away from the enemy. As a minor note, in the same mission I was instantly recognised as an enemy when I approached the base in the stolen vehicle; even when I ensured that I was not detected by driving around and therefore avoiding all enemies.