Dennis,
Start with getting a neoprene and Velcro mask strap, you will wear it out before it breaks. Also, get spring straps for your fins, again, they will not break. Alternatively, for boat diving just get a pair of full foot fins.
As for spare parts, here is a great article, it may be a bit overboard, but it is comprehensive:
Taken from here:
Nitrox, Wreck, Trimix, Technical Diver
Spare Parts and Tools
by Joel Silverstein
Over the past few years I have used every imaginable type of spare parts kit, yet none of them ever really satisfied me. When I was working with my dear friend Capt Billy Deans I noticed that he was wearing this belt pouch, and no matter what needed to be fixed - it was in that pouch.
I was doing an article for Sub Aqua in 1996 and the pouch became the subject of discussion. I had Billy inventory what was in his; I have added some things and subtracted others over time. Take a look at what we carry, and design your own around this system.
The pouch we use is made by L.L. Bean (click picture to go to LLBean) it is a modular system a center pouch with an add-on on each side. There are other companies that make these like Eddie Bauer, and some of the camera bag companies. Find what works for you. The nice part is you can wear it while working, or just carry it on your shoulder when transporting it. Plus, it fits most anyplace.
On the Belt
Titanium Buck Knife in its own holster.
Super Leatherman Tool in its own holster.
Cellular phone clipped onto the belt.
Main Pouch
2 - 9-in-1 hex key sets one in metric, the other for standard sizes
4 tappet wrenches 3/8 x7/16, 1/2 x 9/16, 5/8 x 11/16, 3/4 x7/8.
(The open face and thin wall characteristics of the tappet wrenches make them ideal for working in confined areas)
1 metric tappet wrench 13mm x 11mm for Poseidon hoses
1 adjustable 8 inch Stainless Steel wrench
1 5/16 nut driver for hose clamps
1 needle nose Vise Grip
1 7 inch Stainless Steel Vise Grip
12 nylon tie wraps
1 metric allen key to fit the OMS din to yoke and screw adapter.
Secondary Pocket in Main Pouch
1 hemostat
1 stainless steel pick
1 bronze flat bladed O ring extractor
1 stainless steel suture removal scissors
1 Swagelok caliper
1 small tube of Christo-Lube oxygen compatible lubricant
1 small pad of coarse scrubbing material
1 battery removal tool (25 cent coin) from Cochran and battery polisher
Secondary Pouch Right Side First Pocket
Assorted Sharpie indelible pens regular and fine point
1 white paint stick
1 certification card
1 $20 bill
1 #1 pen with the Little Mermaid wraparound (easier to grasp in cold water).
1 Canon Sure Shot waterproof camera - loaded with 200speed slide film. Perfect for grap shotsand accident documentation, as well as snaping up interesting gear configs, or that once in a life time picture.
Secondary Pouch Right Side Second Pocket
1 calculator
1 small notebook
1 Sunto M2 compass
1 roll of 3M black electrical tape (Dont use the cheap stuff)
1 roll of 3M white electrical tape (Dont use the cheap stuff)
12 inch piece of black surgical tubing
1 8 by 12 inch sheet of waterproof paper
Secondary Pouch Left Side First Pocket
Suntan lotion
1 roll of teflon tape
1 roll of 3M green electrical tape
1 container of mask defogger
1 12 inch piece of nylon cord
Secondary Pouch Left Side Second Pocket
Assortment of adaptors, low and high pressure plugs, wing nuts, flat washers, O-rings for valves, and hoses, a mask strap, and extra mouthpieces for standard and poseidon regulators.