I've recently done 6 tropical dive trips that all had similar limits.
I always try to stay under 4-5 lbs just in case the gear is heavier
from being still a bit wet on the return home.
I tend to take about the same gear as you and a laptop
& charger.
I've gotten it down pretty good now.
I carry on a bag, a small soft camera case, and check a bag.
Carryon bag is about 20 lbs and checked bag will be
about 40 lbs.
The key is bag selection.
For Carry on I use a bag from Ebags.com:
Weekend Convertible
This is the best bag I've ever purchased. I even put my laptop
in a sleeve and put it in this bag. It just works great.
Even has shoulder straps for carrying through the airports.
My carryon bag usually has everything that would still allow
me to have a decent dive trip. Prescription mask, dive lights,
Batteries, regs, computer, 3/2 mm wetsuit, swimsuit, core warmer,
hood, contacts, spare glasses, Sunglasses, glass cases,
any meds, legal toiletry carryon limit, and of course the laptop.
Regs are in a reg bag that drops into the ebag.
Wetsuit and corewarmer are rolled up and layed in.
Mask is in mask box and lights and batteries are placed
in around things.
Camera bag holds the camera, housing, and all rechargeable
batteries and chargers.
It is very small, about 9x6x5 inches.
For a checked bag I've found that a roller duffle works
best for me. If the bag is too large 1 of two things happens,
A) you keep stuffing it until its full and its too heavy
B) you have too much empty space and things slosh around
Also, look around for one that is definitely under 9 lbs.
It amazing how much the weights of these vary.
Mine weighs about 6 lbs and is about 23x13x13.
The other thing to keep an eye on is toiletries.
Things like a large bottle of contact lens fluid, big can of
shaving cream, or large sunblock tube can easily eat up and extra
5-8 lbs over using the smaller sized products.
Also, don't carry many if any extra shoes.
I tend to wear one pair and then pack a pair of Tevas.
As far as packing. I use my clothes as packing material
around things. Save-A-Dive kit is in a case and definitely in
the checked luggage as the scubatool in mine can't be carried on.
Also, keep in mind that on some trips (the bonaire one comes to mind)
that even though you are allowed a carryon, the plane
is so small that there will not be enough room to store a
"normal sized" carryon bag. In these instances be prepared
to check your carryon bag at the stairs of the plane.
When this happens, I'll pull the laptop out of the bag (it is in a padded
sleeve with handles) and then check the bag. If I don't have
the laptop, I might pull the reg bag out or perhaps just
grab the mask case.
Other than the laptop and my prescription mask,
I'm most worried about the luggage missing
the plane rather than being stolen or damaged.
On a trip to Fiji, where a small seaplane was involved,
the weight limit was 25 lbs total! and they gave you extra 12 lbs for scuba gear.
Couldn't take the BC or fins on that trip.
It can be done with out having to really sacrifice leaving
stuff behind. It just takes getting lightweight bags
and not bringing along stuff that won't be used.
Hope that helps.
--- bill