I've got one like this and it's suprisingly stable. I've had about 700 pounds in it too, including the 5 hp merc 4 stroke........

http://www.westmarine.com/buy/west-marine--watertender-9-4-rowing-dinghy--11985504


www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).