[TG] Depth contouring

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I live aboard a yacht and am currently cruising Malaysia. I enjoy exploring rivers, reefs and other shallow areas but taking the yacht into uncharted or poorly charted areas is dicey, especially rivers where visibility is usually poor. I am looking for a device that will help me chart depths from the dinghy. Right now I use an old-fashioned lead with knots on a bit of string to measure depths from the dinghy. Features I'm looking for include:

Portable
Self-contained
Battery operated
Depth below transducer
Position (GPS)
Ability to store depth and position data
Easy transfer of depth and position data to a pc later.

Can you suggest some products I can look at?

Dave.