What you have been told is absolutely correct. Work on technique first and foremost. I have quite a few coaches with various forms of swim education but for most, the main priority is technique. Without proper technique, you will swim on fumes sooner than needed. Just like with most things, once you get the fundamentals down, the rest comes naturally such as the endurance. I asked about the drill work because I wanted to suggest all that you mentioned. Being a former short stroke unilateral swimmer, I found that finger trip dragging helped me to really stretch out my stroke and pace my bilateral breathing.
I was on Cozumel for 6 days (4 days prior to and 2 days after). I left early in the morning to go to the mainland to catch some sights.
Check your mailbox for a PM