To observe sea ice and ocean conditions in the Arctic in summer, a trans-Arctic research cruise of the U.S. Coast Guard Cutter Healy was conducted from 5 August to 30 September 2005. The relationship between the ice concentration observed by the on-board ice-watch and the temperature above the freezing point (ΔT) measured by expendable conductivity-temperature-depth (XCTD) sensors had a negative correlation (CT-relationship) before the onset of freezing. This means that as ice concentration decreases, ΔT increases due to the larger absorption of solar radiation. However, ΔT in high ice-covered regions (>90%) remains more than 0.1 K during the melting season, suggesting that sea-ice and melt-pond areas work as heat source areas as well as leads. By separating the effects of heat input from open water, melt ponds, and ice on the heating of mixed layers, we found that the contribution of the transmitted heat through ponds and ice on the ΔT-gain is large in highly ice-covered regions. To examine the effect of such heating on ice melting, a simplified ice-ocean-coupled model was applied. By changing the heat input to obtain the analyzed ΔT-gain for each surface category, the transmittances of ponds and ice were indirectly estimated as 55% and 9%, respectively. After including the effects of transmitted heat through ponds and ice, the modeled results agreed with the observed CT-relationship. Comparisons between the results of turning on and off the effect of transmitted heat through ponds and ice showed that it amplified the open water-albedo feedback mechanism in the highly ice-covered region.