Is St. Petersburg an icebound port?
Alas, it can be; moreover, even if the lagoon itself isn't, the Finnish Gulf can easily be. As the can Aland crossing of the Bothnian gulf, and the Danish Belts, for example. Key to Britain isolating Denmark from potential allies, the ice.
But here's an example: the Russians took armies across the Finnish Gulf in frozen crossings in 1577, 1581, and 1940, and in 1809 crossed the Gulf of Bothnia. The Kronstadt Rebellion was suppressed by the RKKA crossing the ice of the bay to storm the naval HQ. The Swedes under GA surpised the Danes by crossing the Belts.
Even if not completely icebound, it took sailing ships largely out of commission for the November-March period, variably as the year may turn out. One reason why so many rowed ships were used well into the late 1700s, too.
LA said:
I've also seen sources that the fleet was based in Estonia, so the battle would've been somewhere in the Baltic Sea. How powerful was the Russian fleet at this time? I guess I'm used to later Russian history where there fleet wasn't as powerful, but I didn't think they had a big one.
The Baltic Fleet had 27 ships of the line, some command, about half third rates, some older. Many carried carronades too. There were also about 120 frigates of all sizes, 8 very large ones, and an indeterminate amount of smaller vessels that were rowed, or used as bombard ships.
Revel was the second port they used, but they took refuge in the main base, Kronsdadt in 1809 and I'd expect Paul's navy to do the same. Revel can freeze over too.