Hey 'pup,

Here's a cut and paste from NOAA's Marine Mammal Deterrence Page

"Potential methods for use by fishers to deter Pacific harbor seals and California seal lions from damaging gear or catch (anglers must be actively fishing with gear deployed).

Visual Repellents/Noise Makers:
boat hazing, circling
pounding on hull
pyrotechnics (e.g., bird screamers, bangers, underwater firecrackers, cracker shells)
starter pistols
horns, bells, whistles

Physical Contact:
• slingshots
non-toxic and water soluble paint ball guns
non-lethal ammunition (e.g., rubber bullets, sabot rounds, game stingers)

Methods to Avoid – The following methods and techniques have an increased likelihood of causing injury or death and should be avoided.
No Firearms with “live” (lethal) ammunition
No Devices with Injurious Projectiles (e.g., archery gear, crossbows, spear guns, bangsticks)
No Sharp/Pointed Objects (e.g., harpoons, spears, gaffs, nail studded bats/poles/clubs)
No Entangling Devices (e.g., loose webbing, snares, concertina wire)
No Aggressive Tactile Methods (e.g., striking animals with bats, hammers etc., impact with vehicles or boats )
No Tainted Baits or Poisons

Act Responsibly and Use Common Sense! - Regardless of method or intent, the property owner or fisher may be subject to prosecution should a marine mammal be seriously injured or killed as a result of deterrence efforts for the protection of property, gear or catch."

There should be a run on slingshots and paintball guns at out local sporting goods stores very soon. Hope this helps!