I totally agree with having a Wingman. Having someone close to ya, is an added insurance policy aside from all the safety gear you have on board. Here's my checklist prior to getting underway.

After riding offshore a few times I slowly came up with stuff that I would need.

Things onboard the ski:
(1) GPS and extra fresh AA batteries
(2) 50' 1/2 braid rope
(1) Folding paddle (Orange in color)....doubles as a distress flag
(1) Emergency kit (Flare gun, 5 flare cartridges, signal mirror, dye pack, whistle, magnetic compass)
(1) Extra ski lanyard with Yamaha key)
(4) Bottles of Water
(4) New spark plugs
(1) First Aid kit
(1) Pack of different size SS hose clamp
(1) Quart of oil
(1) Manual pump w/ hose
(1) Extra vest
(1) VHF Radio
(1) Sunblock
(1) Sponge and rags
(1) Sunglasses
Tool kit, vice grip and adj. wrench
Waterproof Jacket and Pants

Things I carry on me/waterpack/vest.
USCG Approved Type III Lifevest
VHF Radio
Cellphone (waterproof baggy)
Protein bars (waterproof baggy)
Lighter (waterproof baggy)
Knife
Whistle
Cash

~~anything else I'm forgetting? Oh, don't forget to tell someone on land about your plans and what time you are expected to be back.