Edit: To the OP, there's really nothing special that scuba-specific rash guards will offer that a technical tee won't do.
Do a purchase tight fitting or normal loose? Just thinking of overall comfort and gear rubbing protection.
Check SPF too. Maybe it is true of all these products, but I know Henderson are SPF 50.
It will do the job, but there are a few other things to keep in mind. There are fitted and loose fitting rash guards. You will find some of the dive ones and all of the surf ones are going to be fitted. Then there are the tech tees that are loose or slim fit (but not fitted) for hiking/travel and then there are the compression fitted ones primarily used for fitness/active sports. The more fitted it is, it tends to layer better under a wetsuit and doesn't bunch as much as a loose fitted one.
I'm a buyer in technical apparel for the outdoor industry and for the purpose of our conversation, there are 3 different levels of smoothness on the face fabric to keep in mind. There are the extra smooth rash guards (like the Excel ones I have), smooth rash guards (like the Roxy onesie I have) and the ones that are a little more knitted feeling (like the Roxy top I have). The smoother ones glide better and layer better under wetsuits than the knitted or almost brushed feeling ones (I have one from O'Neill like this).
You will need to get a compression fitness technical shirt with a smoother face fabric to get something similar to the Excel or dive specific rash guards. The more fitted ones will be more fitted everywhere, including the wrists and neck. Taking it off will be a different process and some find it more difficult - the best way is the manner in which most men refer to as taking off clothing as "feminine" and "seductively".
(Cross your arms in front of you, pull up on the bottom of the hem and over your head, the arms will come up and off naturally.) I do like the fitted ones better for purposes of layering and also because I don't have to worry about the sleeve hiking up the inside of my wetsuit when I pull the arms in and I don't have to hold onto the sleeves at that point, either.
You can use something like the Patagonia Capilene Daily shirts for the smooth-extra smooth in-between and it's between loose fit and slim fit. Wet clothing is not as structurally sound as a dry one when it is being stretched and tugged at repeatedly. This is more of a problem with a more fitted regular tech tee than a loose fitting one as far as loading the seams. A rash guard will probably hold up better in the long run on that front.
UPF is dictated partly by the dyes and the weaving process of a garment. When you take a regular tech tee, the knit/fiber tends to stretch out more when wet compared to a rash guard and that affects the effective weave for the purpose of UPF. A rash guard that is meant to be used in water will hold its shape better and as a result, will keep a tighter knit, thus preserving its stated UPF coverage. You will see "degradation" happen less or none at all in a compression tech tee than a regular tech tee.