TL;DR

dAISy AIS receiver www.wegmatt.com https://www.tindie.com/products/astuder/daisy-ais-receiver/

I made up a 5 element colinear antenna for 162MHz - see the project links for webpage for construction details

I'm using a 3.7V Li-ion battery pack (2x18650) so ideally need something suited to 3.3v operation, especially since the Teensy has an onboard 3.3v regulator already.

Adrian Studer who designed the dAISy has been very helpful with information about low power usage. The receiver can be powered at 3.3v but the power consumption (Watts) is lower when powered via 5V rail for some reason.

More details about the receiver:

http://electron-tinker.blogspot.co.uk/2016/04/remote-ais-receiver-part-2-ais-receiver.html