1. First, three values can be determined: round trip for 3 hours, downstream for 7 km/h, and upstream for 3 km/h.. Rest every half hour 10 minutes, which means 40 minutes is a unit, so 3 hours is 180 minutes. Even if it is evenly distributed, it is necessary to ensure that the time to come back is about twice that of the smooth water (determined by speed).

2. If it is assumed that he will come back in 40 minutes, obviously 100 minutes is not enough. So after half an hour, he can only rest 10 minutes (the title describes that tourists should rest every half hour 10 minutes, and they are not allowed to change direction halfway, so they can only row back after a short rest), so he can only row 3.5 meters at most.

3. Return time: less than 90 minutes, two 40 minutes to row 3 meters, and the remaining half meter needs 10 minutes.

4. The total time is 130 minutes.

Second, when going out to sea, go upstream and return downstream.

1, suppose he returns in three or forty minutes, and draw a distance of 4.5 meters. (120 minutes)

2. It only takes 50 minutes to get back to the actual place, 7 meters an hour, and less than 6 meters in 50 minutes.

1, suppose he returns in four or forty minutes, and draw a distance of 6 meters. (160min)

2. It only takes 20 minutes to return to the actual place, 7 meters per hour, and less than 3 meters in 50 minutes. So I can't go back, so the answer is 4.5 meters.

All units are kilometers.