Sail more, see both!
We had sharks once, on the way from Newport to Bermuda in a millpond-calm sea. Every hundred yards or so in every direction there was the tip of a tail sticking up about 4" out of the water. They would each flop first to one side, and then off to the other as the sharks (basking??) cruised slowly along. When we came up to one, it would sink into the dark, and then reappear a few hundred feet aft.
Whales are much more fun. We were going transatlantic when we came across a pod of about 80 or 100 pilot whales. They lay in the water like telephone poles, with stubby little fins on their backs, and stubby, rounded jaws sticking forward of their heads, making little pfft noises as they jostled around in a big group off our starboard side. A few lifted their heads out of the water and splashed a little. They looked to be as big around as a 55 gallon drum (in the middle) and perhaps 30 or so feet long. They swam near us for about 10 to fifteen minutes, and then melted away.
On another night it was so foggy you could barely see the mast from the helm on our 38' sloop. We were sailing gently along at about 5 or 6 knots, with the bow wave making an occasional swish, when we heard snoring. Very distracting on the 00:00-04:00 watch, with everything else so quiet. Humph. Captain must need turning over, we said to each other, so I went below to give him a poke. Wasn't him; no snoring going on in the quarter berth below. It was louder aft than forward, though. I went to the backstay to listen. It was coming from about 10 feet aft of the transom, with the fog so thick that the stern light's blur only extended about three. A little bit later we noticed an occasional "poof" and swish sound off the leeward side as well. The two (maybe three?) whales "piloted" us through the change of watch, but were gone when we came back on at 0800.
On another day during the trip, a Finback whale gave us a distant perusal, coming up to breathe a few times within sight, several hundred yards off. When he (?) dove, you could see the long arching back and then the small fin, well aft. Didn't see any flukes.
It was really neat to see animals so at home in their environment. They reminded me of cows in a field. Solid, content, purposefully doing their thing and not worrying about it or us. Saw more whales more recently on a trip to Alaska -bubble feeding - but that was on a whalewatch tourist boat so doesn't count here.