I'd make it a point to do whatever it takes to maneuver myself so that I'm not behind Sam.

Or say something to him along the line of "hey man, you know when you coast or drift or reach for your water bottle it makes it real hard to concentrate on maintaining the the paceline". Make sure you're smiling when you say it too. Try to keep it light.

If Sam Squirrel is always a squirrel, I'd make sure he's never in front of me. There used to be a rider with our office group that couldn't ride a straight line to save her life, and even when my lungs were craping out on me, so I couldn't take my turn a pulling, I wouldn't let her back in the line in front of me. Eventually one of the better guys had words with her.

On the other hand, if it's something like Sam veers off when he's reaching for his water bottle or putting it back, then grab yours and drink too. I know I still have trouble staying in the line when I try this little skill.