We use the Cape Coral boat ramp on the weekends because they have lots of boat trailer parking. From the launching area, it'll take you about ten or fifteen minutes to cruise underneath the Sanibel Causeway Bridge. The area is definitely excellent for boating and you can go in all kinds of directions once you get near the tip of the south cape.
Within about a half hour you can easily get around to the backside of Pine Island and Captiva Island for a really nice scenic backcountry boat ride. You have to pay for the boat ramp, there's a daily fee or there is an annual fee for Cape Coral residents. This is a great zone because you have the Public Swimming Pool at the Yacht Club and the Bar Drinkers love the Happy Hours.
Enjoy your boating, as mostly the rivers tends to be rather calm but it can whip up when the winds get over ten or fifteen miles an hour. A nice thing to do is to take the boat over to Doc Ford's restaurant which is extremely boater friendly, but there's plenty of others to pick from that are very convenient to this location.