Never share water bottles. As tight-knit a unit as a crew is, it's all too easy for a bug to decimate a boat during an important week of practice or just before a major race, and sharing water bottles -- no matter what the temptation -- increases the risk. With bodies alternately warming and chilling on the water, legions of happy germs move relentlessly through the engine room. That's the kind of unexpected x-factor you can do without.
Unfortunately, it can be inadvertent. Since most of us buy Powerade or Poland Spring by the case, we arrive at practice with that day's bottle snatched from the supply racked up against the wall of the garage; take a swig or two while stretching to get the hydration going, and toss the bottle and a dry pair of sox into the coxswain's shoe crate with similar others. (Have you ever noticed how everyone seems to drink the blue stuff on Thursdays?) Launch time comes, and there is row of identical or similar bottles in the crate, each with an identical quarter tear in the label to distinguish them. Matching them up again in the hurry of launching -- especially in early morning practices when it is too dark to recognize your own teeth marks -- can be a hit or miss proposition.
Germs also hang out on the human hand, making the oar handles apt transmitters to the next rowers who use them. Many viruses can survive for hours in the air, making it that much more important to wipe the oar handles down with bleach (This should be routine practice to get the blood off of them anyway.)
Coxswains, if you've been coughing all over the mic between calls, make sure you are not sharing. If there is any possibility that you might, wipe the mic down with some of the bleach (or keep some packets of hand wipes in your coxswain's kit for that purpose.)
There are accounts of 17th and 18th century merchant ships running up the yellow flag with the black ball* to discourage pirates from attacking, but pinning one over your bow number while your crew is coughing and retching over their oars probably won't intimidate your opposition with the same degree of effectiveness.
Stay healthy, stay fast.
* still the international signal for "I Have Fever Aboard."