When we've been on Maui in early April, we've always seen whales from either shore or a boat, and heard them underwater while diving, but we've never seen them underwater.
We dive with Lahaina Divers most of the time (we know a couple of the DM's well and are always treated like family) and actually like the long boat ride to Molokini. The ride is usually smooth and the views are gorgeous, plus we get to whale watch during the season.
It IS a long boat ride, so if you don't like long rides definitely drive down to Kihei and hook up with B&B, Mike Severns, or Ed Robinsons.
"Turtle Reef" refers to an afternoon charter that sticks close to shore. The afternoon trades come up and make dive conditions less than desireable in open areas. There are a bunch of sites they go to up and down the Maui coast. Very often Lahaina Divers will take you to Mala Wharf for one dive, and it's a dive that I really like. It's a huge wreck of a wharf in a fairly protected area that provides great views of all sorts of sea life (turtles, sharks, and oodles of fish), and also has some fun swim thrus.
Lanai is definitely one of my favorite places to dive. It's likely that you'll dive either Cathedral 1 or 2 on your charter, and that's a definite plus! The Cathedrals are gorgeous and amazing to dive, but remember to take your flashlight. We didn't on our dives in August and really regretted it, but we got some fabulous pictures of the light streaming through the cathedrals, so that was cool.
When will you be on Maui? We'll be there from April 1-6, and will likely be diving with Lahaina Divers 3 or 4 days.