The advice on the 3' whip antenna is good. Having a longer antenna with more gain won't gain you much, if anything.
The advice on connector problems is right on the money
. Over the last 15 years of marine electronics
work, almost 80% of the PL-259 connectors are poorly assembled.
Check the cable type. If you have the small RG-58 wire, even under perfect conditions you will lose about 75% of the power over a 100' run.
Finally, measuring the VSWR is often NOT a good indication of whether the antenna is working. The SWR measurement only works properly when performed at the base of the antenna, not very practical when the antenna is 75' up. Losses in the cable (see previous item) will make the SWR measured at the radio
look good even if the antenna isn't working at all.