Assuming you dont know snails location, you cant be ever sure to be safe even if its extremely slow.

What if you happen to be close enough without knowing its there?

How would he recognise it? 

What if he was doing his job poorly? 

Motherfucker better not leave a slime trail all over my waifu pillows. 

Also, what's stopping you from going to the other hemisphere? The ocean is so vast and has so many crevices that the snail would take years to cross.