Help me find an ultrasonic sensor for a DIY echo sounder (marine use)?

I've been crawling through the web searching for something (affordable) I could use but I can't seem to find anything reasonable. Pretty weird as you can get a cheap $35 fish finder on Ebay. The sensor would need to be in-hull mounted meaning that the sensor would need to be able to ping through the hull (which the aforementioned fish finder does).

I guess I could buy the fish finder and a) use it (boring) or b) use its sensor (not sure how).. I don't need a huge range, something around 30 m / 100 ft would do fine. Ideas?

No idea how to build a depth sensor for marine use?

frasco:
No idea how to build a depth sensor for marine use?

First time I’ve seen this question. Which fish finder? Any links? I’m sure someone would be intrigued to check out the fish finder spec and offer some suggestions. BTW, I didn’t know fish finders are just ultrasonic ping sensors so thanks for posting.

The link to the fish finder is in the OP. If for some reason the link doesn't work, here it is again: http://www.ebay.com/itm/320740620025?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649#ht_4717wt_1270

Not sure about the tech, if it's just an ultrasonic ping sensor, as I'm not that knowledgeable about electronics - yet. ;)