Being comfortable while diving is a relative term. For some, being comfortable means diving in warm water with great visibility and no current. For others it might mean having equipment that functions well and that fits well. Sometimes it is hard to control the environmental factors. However you can do a lot to make your dive more comfortable by making your equipment as comfortable as possible.
One of my students had all kinds of problems with the mouthpiece of the regulator she was using during the certification dives. The equipment problem almost prevented her from completing the certification. Eventually we were able to get a better fitting mouthpiece and she had no further equipment problems.
Recently I got an email from her letting me know that she is going to purchase a mouthpiece that she knows is comfortable and will be taking the mouthpiece on her upcoming dive vacation. She will use her own mouthpiece on the rental gear at the resort. I like the idea of being proactive in dealing with gear issues.
There are many things that one can do to make your gear more comfortable. For example, making sure that you have adequate exposure protection (to prevent sunburn, abrasions and stings). Another example is making sure that you can operate all the various releases, inflators etc. on your BCD. This is especially important when you are using a rental BCD. On your checkout dive at the resort, it is good idea to practice inflating, deflating, etc. without looking. You should be able to locate the low pressure hose, find the correct button and inflate/deflate without looking. You should be able to find and use any air releases on the lower back of the BCD (if so equipped).
Being familiar with your gear, and making sure that everything fits and feels good on you will go a long way to making you more comfortable in the water.