An antenna really doesn't care anything about what it is receiving, it just collects electromagnetic waves in the frequency range for which it was designed. VHF radios and AIS both operate in the same frequency range. They use simple dipole antennas which are non-directional in the horizontal plane. Dipoles are inherently broad band. The gain can be changed by adjusting the dipole length which simply narrows the vertical width of the beam. You cannot increase the gain by making it "narrow band" for a particular frequency. There is a lot of snake oil sold in the antenna world, just look at TV antennas. Don't spend extra money to buy a dipole antenna that that purports to work best for some particular purpose, like digital TV or AIS.
Paul