Just saw post. As a cold weenie and anti-struggling into tight things, may I suggest the following. I have little body fat as well.
First combination (my jack of all trades)
I use the following Or variations thereof for Roatan, Utila, Coz,Thailand, Philippines, Brazil (temp from mid 70's to hi 80's).
1)Hood always. silicon swim cap+ Neoprene that triathletes wear or neosport 2.5 mm beanie
2) 3 mm aqualung shorty + 3 mm Henderson vest+ long Rashguard (occ two rash if night dive and if feel tired n chilled)
3) long swim leggings- Swim leggings depending on time of day or temp or how I feel varies from ordinary to polypropylene (from leisure pro) to $ 20 odd 2 mm neoprene pants for sailing I think which I got cheap fromAmazon. Gives me flexibility.
I have basically stopped bringing my long 3 mm suit.
As my shorty has loosened with time I started wearing vest UNDER as opposed to over shorty. My core seems to keep warm better this way.
Second combination for colder temps
In June in GreatBR-Lavacore hood, 5 mm aqualung+3 mm Henderson hood+ Long rash and long leggings from Leisurepro. I was VERY comfortable. I have never used this set up ever since then but would recommend for temps in 60s.
Third bad combination
In Houston TX where I certified in low 60s in March, I think I had anywhere from 12 to 14 mm combination and still felt cold. I shall NEVER inflict this torture on myself ever ever ever again.
I get cold sooo easily that I try other warming technics depending how chilled I am.
1) dry off or change into dry Rashguard between dives
2) if lazy, wear raincoat or plastic bag over myself
between dives. Plastic impenetrable against wind.
3) when boat stops moving and no wind. basking in sun works best!!!
Layering prevails in my combinations. I credit my clothing choices underwater to years of living through Chicago winters. I layered and was able to survive 30-40 degree variations in weather throughout day, indoors, outdoors etc. Hope some of my experimentation with gear helps a fellow cold weenie out there.