A set of jumper cables will do for the test on the ground system ya know car jumper cables. Also get rid of the volt meter and use a test light, for those that are unsure or learning about DC electrical systems the volt meter can be deceiving can't go wrong with a test light and as far as the low voltage issues if the batteries are at 12 volt it should engage the starter and turn the motor over low voltage will burn up starters but a solinoid is just a switch that is turned on or engaged by a electromagnetic coil that engages when voltage is applied to the coil. If you start at the posi. term on the starter with the test light and go backwards stopping and checking every connection of that wire until you get to the positive terminal on the battery while the key is turned to the start position you can rule out that and move on then do the same with the ground by connecting the test light clamp to the pos. side of the battery. If your on the boat and can't figure it out call me I can probably talk you through any checks you'll need to do 281-850-9521.

Good luck
Scott