Up here in the not very tropical Pacific Northwest, paddling guides tend to carry 'Hypo Kits' to supply their students when they get cold, fatigued, or seasick. Hype = Hypothermia. When you're guiding or instructing you never know how the day will go. While it's your job to keep your students safe, sh.. happens, and you want to be ready for anything.
One thing to keep in mind are those students who hold back their real feelings about their body temperature. Guys in particuliar usually can't admit to being cold or fatigued.
Here's what I carry in my Hypo Kit. The items are stored in an Outdoor Research double dry sack under bungys or a cargo net attached to my SUP deck (or day hatch in my kayak). Kayak deck bags are also great for storing such gear.
-chem heat packets
-small vhf radio (Icom waterproof floating version)
-xtra fleece lined hood (full or half hood) or fleece hat.
-xtra fleece lined gloves in sizes relvalent to those in the class
-power bars or similar
-compact mylar emergency blanket (folds into a few inches)
-simple first aid kit (band-aids, advil, aleve, etc.)
-xtra waterpoof headlamp or paddling light (and extra batteries).
-nylon rain shell (L or XL to fill all) and pants.
-drinking water (carried separately).
-sunbock. (and sunblock lip baum).
For Warm Temperature Areas consider carrying the following:
-wide brimmed hats to cut out sun.
-UV rated long sleeve cotton shirt.
-simple first aid kit.
-solarez for dings.
A smaller version of the above can be kept in a kayaking style type 3 pfd pocket (vhf, power bar, hood, 1st aid, etc).
Check out tow systems from Kokatat and North Water.