This my third Nekton trip this year so not expecting too much new
Things I have learned:
Bring many bathing suits I take 4 - and pack lightly re everything else you won't be wearing much else.
I pack three wetsuits, two would be enough, then you are never getting in to a wet wetsuit. Makes a difference re feeling the cold during the day. Instead of being wet almost all day you are only wet when you are in the water - not while gearing up. Makes a difference for me.
Bring an extension cord with lots of outlets. The placement of the single plug (two outlets) in the cabin is not optimal and depending on what you need to plug in you might not have enough with only two.
Soft luggage is a good idea, then you can unpack and push it under the bed for the trip.
Relax, no need to jump in as soon as the dive deck opens. Hang back, let the crowd clear and then get ready on your own time, you will still have time for the second dive.
That's about it - the diving is easy, the crew so far has been excellent, the boat is easy to dive from and has lots of space to hang out in.