The First Battle of Svensksund — known to Russians as the First Battle of Rochensalm, after the Finnish word for the strait, Ruotsinsalmi — was a fight unique to the Baltic. The water here is too shallow and too rocky for ships of the line. Both sides built specialized 'archipelago fleets' for it: shallow-draft frigates with strange names like turuma, hemmema, udema, and pojoma; gun sloops and mortar longboats; oared galleys descended directly from those of ancient Greece. King Gustav III's Sweden fielded one light frigate, a handful of these archipelago frigates, twenty gun sloops and a scattering of galleys, with about five thousand men. Catherine the Great's Russia brought a much larger force — a frigate, eight xebecs, eighteen galleys, twenty-nine half-galleys, and supporting ships totaling around twelve thousand men, with a separate squadron of three frigates and other ships under Ivan Balle in support.

The Swedish commander, Carl August Ehrensvärd, had a sound plan but a bad king. He wanted to scuttle ships across the narrow passages between the islands, sealing the channels through which Russian forces could approach. On 23 August, his crews began sinking blockships in the choke points. They missed one. Ehrensvärd judged the strait between Tiutinen and Koiromsari too shallow to navigate — a small mistake that would cost a fleet. King Gustav III had also overruled key parts of the plan, refusing to let Ehrensvärd withdraw to Svartholm fortress after dealing with Balle's squadron, even though everyone aboard the Swedish flagships knew their ships were running low on ammunition for the longer fight that was coming.

The action began at ten in the morning on the 24th. The first phase, against Balle's supporting squadron coming up from the south, went well for the Swedes — six hours of artillery duel, three Russian ships captured, others badly damaged, the southern attack beaten back. Then, at four in the afternoon, the rest of the Russian coastal fleet arrived from the east. Litta's galleys threaded the supposedly unnavigable strait. Suddenly the carefully scuttled blockships meant nothing; Russian crews could clear the rest of the obstacles from the safe side. Swedish ships defending the line were forced to fall back. The turuma Sällan Värre ran aground while fighting and was captured. Enrique MacDonell's hemmema Oden, coming to her aid, was taken too. Captain Ehrensvärd's own flagship, the turuma Björn Järnsida, ran aground and struck her colors only after fighting nearly to the last man. The frigate af Trolle and the turuma Ragvald followed her into Russian hands.

The final tally was brutal. Three turuma archipelago frigates, one hemmema, one light frigate, a galley, a half-galley, a schooner, nine gun sloops, and thirty transports and auxiliaries — gone, captured, or burned by their own crews north of Kotka island to keep them out of Russian hands. Roughly fifteen hundred Swedes were killed, wounded, or captured, with another five hundred sick men in a hastily formed field hospital, too unwell to evacuate. Russian losses were astonishingly light by comparison: one galley and one gun sloop blown up, 383 dead, 628 wounded, 22 captured. The ships Balle had lost early were all recovered. The shallow channel that Ehrensvärd had not blocked was, by every honest measure, the single deciding factor of the day.

The story does not end there — and that is what makes Svensksund unusual in naval history. Almost exactly one year later, on 9 and 10 July 1790, the Russian and Swedish fleets met again in nearly the same waters. That second battle, the Second Battle of Svensksund, became the largest naval engagement ever fought in the Baltic Sea and Sweden's greatest naval victory of all time. King Gustav III, in personal command, destroyed roughly a third of the Russian fleet and saved Sweden from a disastrous peace. The two battles together — defeat in 1789, triumph in 1790 — turned this stretch of skerry-strewn coast off modern Kotka into one of the most concentrated patches of naval history in northern Europe. Today the seabed here is a protected marine archaeological zone, with the wrecks of dozens of warships from both navies still lying among the rocks where they sank.