Yes to the hand held GPS & VHF. As Capt. J said,"It depends on the boat and how hard it is to get parts". It also depends on your usage and the crews ability/willingness to do repairs. If you're heading for Antarctica, one of everything. If you're doing day cruises or island hoping and a breakdown and waiting for parts only means staying dockside for a few days not so much. Do you have an engineer or are you seriously into fixing your own stuff? If you're the type to just make emergency repairs to get you back to the dock where you'll call in the mechanic then I'd recommend spare fuel filters, belts, light bulbs, spare batteries for all flashlights, radios,etc., electrical tape, 'Atomic Tape' in case a hose breaks, clamps, cable ties and a spare starter. Also, a block of wood to keep the shaft of a down engine from spinning. From there build your list depending on need, space, ability, willingness and wallet. As a general rule, when something breaks I like to have it replaced and the broken item repaired and kept as a spare. It's a safe bet that when you break down though it will be the one item you're not carrying.