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.