Since I am an attorney, I suppose I should chime in on this one.
When developing a product that competes with another product, first ask yourself this question: what protection does the product enjoy (e.g. patent, trademark, copyright)?
If the product is patented, then you need to look at the claims contained in the patent. In general, the only product features protected by a patent are those disclosed in the claims. It's easy to tell what the claims are--the patent itself says "I claim the following: (1) the process...etc." Once you've determined what patent claims you're dealing with, then you can look at your proposed invention and compare it to the claims. Generally speaking, if your invention is not the same as the patent claims, then you're probably O.K. However, if your invention does substantially the same work in substantially the same manner, and accomplish substantially the same result as the patented invention, then you can be held liable for patent infringement under a legal theory called the "doctrine of equivalents." See e.g. Sarkisian v. Winn-Proof Corp., 697 F.2d 1313, 217 U.S.P.Q. 702 (9th Cir. 1983).
Although you are correct that a serial cable and a couple of interfaces probably are not protected by a patent, a method or process of connecting them or wiring them in a special way could be patented. I know it sounds sort of crazy, but it happens. One example of a crazy process patent is the process by which tape is applied to drywall joints and corners in a particular fashion. Researching which patents may apply to your invention, the components or the use of the components is simply part of the due diligence work you should consider performing.
Trademark and copyright are probably not big concerns for your invention. There really isn't anything wrong with using a trademark for comparison purposes--like saying "ours is cheaper than Suunto's (TM)." And unless you're stealing their operating manual or using their materials in some fashion, copyright issues probably will not arise.
Now that you're past the intellectual property inquiry, you move into the realm of product liability. Ask yourself the question: what can possibly go wrong? This is the time for you to engage in paranoid, delusional brainstorming. No matter how far fetched, write down the possible consequences of using the product. Then divide the list into the realms of "reasonable possibility" and "sheer fantasy". You might want to consult with "experts" to determine what events are reasonably possible. Generally speaking, if you get sued for negligence, you will be held liable only for those events that were reasonably foreseeable.
Once you're aware of the possible problems, build your invention in such a fashion to avoid those problems. You may need to make a business judgment at this point if it will be too costly to build the invention in a way to avoid the problems. The classic case of this sort of business decision is the Ford Pinto. Ford knew that if the Pinto was struck in a collision in a certain fashion, the gas tank would explode and likely kill everyone in the vehicle. Ford even had actuaries calculate the probability of the particular kind of collision occurring and the probable cost if Ford were sued over the accident. Ford calculated that it would cost about $20 (or some other de minimus amount) more per vehicle to make the Pinto immune to the problem, but the probability of the event was so low that it did not make financial sense to spend the extra money. Thus, we got the infamous exploding Pinto.
I'm not suggesting that you follow Ford's example, but be aware of the possible exposure you will face if something goes wrong.
If you get sued on a theory of "strict product liability", then you're out of luck unless you have a nifty disclaimer that puts the user on notice that s/he assumes the risk bad things happening. That's the reason you see the disclaimers saying that you realize you're nuts to be relying upon a particular piece of equipment and you assume the risk of all sorts of bad things. Assumption of the risk is one of the few defenses to strict liability.
A related issue is warranty coverage. You should look at the warranty on the various computers. If it says that use of any non-Suunto/Uwatec/Source/Whatever equipment voids the warranty, then you need to be aware of the fact that if a computer fries because your interface was jiggy, you can get sued. Again, this is the place for warranty disclaimers. Disclaim all the warranties you possibly can, particularly warranties of merchantability and fitness for a particular purpose. Different states have different laws concerning which warranties you can legally disclaim, so check with your local lawyer.
These are the issues that spring to mind immediately. I hope its helpful. Good luck with your project.