Paddlesport-specific PFD's are much more comfortable (making them more likely to be worn 100% of the time) than other styles or brands of PFD's. Some states have additional requirements for PFD's. For example commercial rafters in Colorado must wear a Type V PFD (the guides can wear a Type III).
Some good PFD manufacturers include:
PFDs for sea kayaking and for whitewater paddling can be different even though they look very similar.
Rescue and towing harnesses:
With appropriate training, a rescue PFD is highly recommended for whitewater. Rescue PFDs have features designed to enhance wearer's ability to rescue other paddlers. These include such features as attachments for pigtails and a quick-release chest harness.
Some sea kayak PFDs have similar looking items, such as towing straps that resemble a chest harness. Note that these are very different and you should not use a sea kayaking PFD as a whitewater rescue PFD. WW chest harnesses are designed to carry a load of 500 kg (1100 lb) - towing belts are not. Chest harness straps have braking bars to take up load - towing belts do not.
It is reasonable to use a whitewater rescue PFD in sea kayaking, however.
Another aspect of the difference is flotation. Whitewater PFDs come with different amounts of bouyancy. Standard in Canada and the USA is 15.5 lb, if I remember correctly, while some rescue PFDs are 17.5 or 18 lb. If you are particularly lean and muscular, you might appreciate the extra bouyancy of some of these rescue PFDs. That said, I believe that there may be higher bouyancy sea kayaking PFDs as well. Consider as well the bouyancy of neoprene wetsuits.
Note that sea kayak PFDs may have more reflective material on them than whitewater. Sea kayakers expect to be seen on the water, especially at night. If you get a whitewater PFD for sea kayak use, you could add some adhesive-backed Scotchlite or similar reflective tape. For better visibility in general, international orange is the easiest to see on the water. Bright yellow comes next. Most other colours are harder to see.
Sea kayak PFDs often have (and need) (more) pockets, for things like flares (which should be on the person, not on the boat), hydration pack, snacks, VHF radio ... Whitewater kayakers can usually stop and get out onto the bank for food and to help others, whereas sea kayakers may remain in the cockpit for many hours.
Most jurisdictions require paddlers to use only certified PFDs. If you modify the PFD, technically, the certification is voided. Sewing, cutting, or any other actions to modify should be avoided.