BUCHAREST -- Romania's Grupul de Salvare Marina ship salvage group has signed an $800,000 deal with a Greek firm to refloat a wreck that has impeded navigation on a main Danube channel since 1991. "We've signed an $800,000 contract with Greece's Drossinos Elefterios to refloat the Rostok," Alexandru Dima, head of the state-run Grupul de Salvare Marina, or GSV, said by telephone last week from the company's headquarters in the Black Sea port of Constanta. Under the contract the Greek firm would remove the wreck of the Soviet vessel Rostok from the main Sulina channel on the River Danube during the second half of July, Dima said. He said the terms of the contract had been endorsed last week by both shipowners and a London-based insurance club which had insured the vessel. In September 1991 the Sulina channel in eastern Romania, a main waterway through the Danube delta to the Black Sea, was closed when the Rostok struck the banks, broke up and sank with a cargo of 5,400 tons of steel coils.