Theres no time limit on the swim, if its a general fitness issue then you might question the safety of splashing around 18metres down with a current.
If you want to look at pretty fish in relatively shallow water i wouldnt get too stressed about it.
Be wary of boat dives on offshore reefs. Theres more than a couple of divers who got left behind. The current at depth can be very different to the surface.
I heard of two german tec divers who were left behind 7 miles off Phuket a couple of years back. Their smb failed and with a two hour deco off the line they were several miles away from the boat when they surfaced. Took them the whole night to swim ashore!