If anyone else wants to rent gear between Cancun and Tulum, I'll include my findings and methods here.
In pursuit of finding a shop that would rent gear, I sent emails to
many shops. I was primarily interested in the area between Cancun and Tulum for Dec 23, 2018 - Jan 1, 2019. Things may change in the future. To find shops, I searched on Google for "<location> SCUBA shop".
Most shops below provide gear for clients who are interested in committing to either guided tours or other classes and those shops tried to up-sell us on such services. We were not interested in these other services.
The only shop that would rent gear without hassles was:
UnderworldTulum
The following are the shops that I contacted that would
NOT rent gear (this is in no way a reflection of their other services, only that they would not rent gear to an insured, certified PADI instructor):
Akumal Dive Center
Fantasea Dive
Diversity Diving
Dive Cenotes Mexico
MexiDivers
ScubaCancun
Protec Dive Center
Blue Life
Aquanauts Dive Adventures
Solo Buceo
Akumal Dive Shop
Aha Scuba Diving
DIR Mexico
Divers Underground
Cancun Diving
Deep Life Divers
Divers Point
ScubaMobile
Akumal Bay Diving
Cancun Scuba Center
Tankah Divers
Infinity 2 Diving
Scorpio Divers
Always Diving
Beyond Diving
Aqua Excursions
Abyss Dive Center
Wet Set
Squalo Cancun Divers
Reef Quest Divers Mexico
Me Dive Center
Playa Scuba
Dive Shop Mexico
Mexico Blue Dream
Alberto's Diving
Wet Owls
Koox Diving
Dive With Ruben
Om Delfin
Quelonian Divers
Seth Dive
Third Dimension Diving
Scuba Total
Tulum Scuba
Scuba Freds
CenoteExperience
Zen Diving
The following are the shops that I contacted that
DID NOT RESPOND at all:
Dive Tulum
Dive Cenotes Mexico
Bahia Divers
Scuba Diving Cancun
Cancun Scuba
Cancun Diving Adventures
Caribbean Connection
Diving Cancun
Scuba Playa
Scuba Diving In Playa Del Carmen
Marina Dive Center