We have developed a remote operation support system for underwater excavators and conducted Sea experiments. Underwater excavators have been operated by divers boarding on them. Their main task is levelling rubble mounds of port structures in Okinawa?Japan. However, in recent years, remote control is required due to the decrease in the number of the divers. Therefore, we have been developing tele-operation support system for underwater excavators. This system integrates three elemental technologies such as "Underwater Information Presentation" that serves as a display interface, "Measurement Method of Mound Shape" and "Attachment for levelling works operated with simple input". So far, we conducted elemental tests of each technology underwater and on land to confirm their usefulness. In this report, we performed experiments using this system in the sea. First, the elemental test of the attachment was performed by boarding operation of divers. The purpose of the attachment is to perform levelling works without difficult input for position adjustments in order to improve the work efficiency of remote operations. The purpose of the tests is to evaluate the performance of the attachment alone. The measurement items are the varies of mound height and the working time. As the results, the attachment was confirmed to be able to submerge the mound and the operability in the sea is the same as that on land. Next, a remote operation test simulating levelling works was conducted to measure the work accuracy and the work efficiency. As the results, by adding a mechanism to move stones to depressions, this system is demonstrated to level the mound from unevenness of ±30 cm to that of ±10 cm. In addition, it is confirmed that the working times with the system depends on the operators' experiences of tele-operations more than that of boarding operations. From the above, the proposed system is demonstrated to be useful for levelling works of mounds by tele-operated underwater excavators.