Is there a list of usable gear for the surfing missions posted somewhere? So far I've figured out that knives and tasers are usable
Use Stats For Nerds (hit "INFO" on a weapon's ufopaedia page); weapons have categories/tags on them that will tell you what missions they can be taken on. Weapons that you can take to the Beach should be marked "All Beach Gear".
Options include:
Nonlethal Ranged- Dart Pistol/Rifle, Taser
Lethal Ranged- Crossbow, Flare Gun*
Melee- Machete, Combat Knife, Knife, Shiv, Master's Cane, Golf Club, Baseball Bat, Club, Boombox, Surfboard, Extinguisher
Other- Flashlight, Flare, All healing items EXCEPT Healing Gel
*Technically lethal, as it does 20 incendiary damage with a small radius. It will almost certainly panic the target if you can score a hit. The crossbow does quite a bit more damage though, in good hands.