Look for one that has or will accept an external antenna; an internal antenna may not work under the roof of your houseboat. Also look for an NMEA output connection, so you can connect it to other devices (computer, VHF, autopilot, etc). Those requirements will thin out your options considerably.
I use an old laptop running off a Garmin 76 handheld. The NOAA charts are free and easily updated, and the laptop gives me a nice big display. I can even make my own charts. I can also remove the handheld and take it in the dinghy with me to go "scouting".
It works for me; though I did have to connect an external antenna as the houseboat roof blocked the signal.
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw
07:01:29 AM by