I did read the question. He said, "1 tank". He didn't say what size tank. He also didn't say what depth. He merely inquired about whether it was doable. You state it's 2.5 miles to the reef, then 50 minutes around the reef.
That's completely doable.
Imagine diving an LP121 with a resting sac rate of .3 (because you're on a scooter, you're not swimming at all). You are at 15' which is approximately 1.45ata. 1.45 ata x .3 = .44cu' per minute. An LP121 holds 165cu' of gas. 165cu' / .44cu'/minute = 375 minutes. Now, the good Magnus has a speed of 250' per minute at top speed for 5 miles or 2 hours. The island is .5 miles in diameter and is 2.5 miles away. So, that's roughly 4 miles including the trip to the island and all the way around the island. SO! Can we do 4 miles on the big bad magnus before we reach 375 minutes worth of gas? YUP!!!
