Medicae is the signature skill of the Apothecary. However, the healer role is generally a less popular choice in RPG (in my experience). Given the nature of the Deathwatch game the role becomes both less attractive to individuals and more important to the team.
Medicae is a Rank 8 General Space Marine Advance (800 point), so by RAW it is out of scope for any non-Apothecary (less than Rank 8!). However, why is basic training in Medicae so rare amongst general Space Marines? It could be argued that it might be SM tradition, but I don't know if there is any canon on it. It seems to me that a Killteam without an Apothecary is at quite a disadvantage - they have no method to recovery any wounds. Also geneseed cannot be recovered.
An option for the GM could be to allow another specialist to spend 800 points on Medicae at character generation. If he has 10 req point to spend on a mission, he can even equip an Narthecium. He doesn't get the special bonuses for an Apothecary of course, so he is not as good. He cannot train it further (without GM agreement) either. However, having this character about will allow some recovery between combat encounters.