Second Skin is a great solution if your blister is a few days old. This product, which you apply to the blister with a small brush, dries over the wound creating a second skin, allowing you the freedom of a painless foray into the water. This product should only be used if the blister is a few days old, as the label suggests.
Remember to monitor your blister in the days before a race and snip off the dead skin surrounding it prior to competition. Although it is never recommended to cut away the skin soon after the blister pops (that may result in infection), after a few days it is safe.
While it still may sting underneath, the removal of the loose skin will eliminate the distracting flapping you might feel under water.
Chafing is the most easily overlooked Worst Case Scenario that undoubtedly can cause the most grief. Chafing occurs in salt water, where areas of your body rub together and create sports hickeys that can last for days (and sting throughout the rest of your event).
Common chafing areas are the underarms, neck, and around swimsuit straps and openings. Chafing also occurs if you wear a wetsuit, mostly around the neck or armpits.
Vaseline is an easy solution to chafing, and any serious open-water swimmer never packs a swim bag without it. A small amount rubbed around the susceptible areas is all you need to avoid chafing, though Vaseline is not recommended if you use a wetsuit (the petroleum jelly can damage the rubber and cause it to deteriorate over time).
There is a great wetsuit-friendly lubricant on the market that triathletes and surfers swear by, called BodyGlide. Found in most sporting goods stores and surf shops, BodyGlide works as well as Vaseline, without the greasy residue. It also comes in a convenient roll-on stick (like anti-perspirant), with none of the mess that results from the manual application that Vaseline requires.
Open-water swims in the ocean can be a lot more frightening should you face a set of 10-foot shorebreak when the gun goes off (and any sensible race director will consider postponing the race should that occur).
In the event that you find yourself facing down a Perfect Storm-sized behemoth of salt-water force, your first instinct may be to swim over it.
The smartest way to avoid a breaking wave is to dive directly under it. If possible, dive to the bottom and pull yourself forward by grabbing the sand. This serves two purposes: one, it lets the wave pass overhead and safely keeps you out of range of its pull; and two, it allows you to use the ocean floor as leverage to pull yourself forward while less-seasoned competitors get whitewashed and thrown back.
As frightening as waves look, their bark is always bigger than their bite from the oncoming swimmer's point of view. Underneath, however, the water is quiet and still, and ideal for bypassing the rush above.
Of course, the above Worst Case Scenarios seem rather quaint for those swimmers who may have encountered a shark or Portuguese Man-Of-War. However, they are common horror stories that require very little of you should you wish to avoid them in the future.
Pre-awareness is the most important step in avoiding these pratfalls and having a great race.