Tandem boats, such as canoes and 2-person kayaks, are often referred to as "bicker boats". There are two camps on this topic: the "couples that paddle together stay together" camp and the "sleep tandem, paddle solo" camp.
One thing we've found to be true. If you paddle together it's either going to enhance your relationship or be the end of it (in the boat, anyway).
When you are away from the distractions of daily life you slow down and pay more attention to where you are and who you're with. This is one of the best things about paddling a boat. It also brings your relationship to the forefront, and anything going on there will be exacerbated, for better or for worse.
Paddling brings you into a team situation that almost always provides unpredictable challenges - whether it's simply keeping the boat going in a straight line, or navigating tricky current or waves. This can amplify any hierarchy present in the relationship and/or lead to blaming your partner, which is a dangerous road to go down. "Didn't you see that?" "What were you thinking?" "If you hadn't put that (insert stroke here) in, we wouldn't be in this mess." Or the opposite: "If you had put a (insert stroke here) in, we wouldn't be in this mess."
One wise fellow we know told his daughter not to marry her partner until they had gone on a two-week canoe trip together. "A long canoe trip is a pretty good indicator of how marriage is going to be after the honeymoon period wears off," he says. (The daughter did actually take her partner on one of the roughest bush-whacking trips she could think of, and they are still happily married 14 years later.)
Another thing we've found is that there doesn't seem to be a correlation between skill and relationship outcome when paddling together. It's natural to assume that skilled paddlers should have fewer issues as they can better control their boat - but with this comes higher expectations, so it all evens out.
We know couples who love paddling the same boat together. We also know couples who start fighting the moment they push off from shore. For some, two solo boats is the best way to paddle together and keeps them (and everybody else around) far happier.
Our best advice when in a boat with your significant other? The tip we all learned in Kindergarten: if you can't say something nice, don't say anything at all.