As far as nitric oxide supplements go, I think they are useless. They were about 95% marketing, and maybe 5% or less results. Now, actually over the last 6-7 years, the market has become flooded with cheaper knock-offs. NO2, the original product by MRI, had a fantastic marketing campaign but for most people was a waste of $80-100(it's still ridiculously expensive even now).
I don't think there are any health problems related to using a "nitric oxide" supp. Just as well, you do understand that you don't actually take "nitric oxide". Most products contain ariginine or some form of arginine and other amino acids that act as precursors and help facilitate the production of more nitric oxide in the body. SO no, you don't take some NO2. You take arginine and in theory it's supposed to make you produce more nitric oxide which leads to all the things whatever the BS advertisements claim. In practice, it's a waste of money.
Supplement-wise, if you want to take something, look into a beta-alanine supplement, especially for endurance stuff. MAN Sports is a great company. Body Octane and Clout are great BA supps. I also like Jack3d by ESP labs. It's just caffeine, beta alanine, creatine, and some other aminos. Purple Wrath (BA + BCAAS) by Controlled Labs is really good to. Controlled Labs makes some god products and is a good company. There are other good products out there as well. But "NO" products are usually a waste
TRAINING: For SEALS, you don't really have another option but to train for it. Even though Primal doesn;t advocate it, you won't make it if you don't train for it. Minimalist Primal fitness won't prepare you for it. The only way to get in condition for SEALS is to train for it. A lot of running, bodyweight stuff, etc, done frequently.
You can still eat Primal of course! Just make sure you are eating enough and resting enough.