Lojsta Lakes
The five lakes in Lojsta are interconnected, and they were likely even more so before the marshlands in the area were drained sometime in the 19th century. There might have been a connection extending all the way to Sigvalde Träsk in Etelhem (another deep lake with a swimming area) via Hageby Träsk.
There is a small dock, and it's perfectly fine to swim or paddle a canoe on at least two of the lakes. A bit further out on the dock, you can see how the lake becomes deeper, at least ten meters. Those who enjoy fishing can purchase a fishing permit.
A bit ahead on the right, there's an opening into Rammträsk, where you can paddle along the cliffside and discover small caves.
Broträsk is located right by Lojsta Slott, which is historically interesting, although it is not a castle in the baroque sense. It was a type of fortress, of which quite large embankments still remain.
Waterways
Sigvalde Träsk, the swimming area in Etelhem, seems to be connected to Broträsk in Lojsta through channels that originate from the rocky terrain between Buttle and Etelhem. These channels flow down through Barmyr (Bara Myr), continuing a few hundred meters south of Etelhem Church and then split into Tänglings Myr. One of these small channels appears to connect with Hageby Träsk, which in turn, through another small channel, links to Broträsk.
A few hundred meters from the point where the canal in Tänglings Myr divides lies a substantial ancient fort called Bendesborg. Its exact age is unknown, but it's estimated to date back to the centuries around the time of Christ. The fascinating aspect of Bendesborg is that the lakes in Lojsta, through channels and streams, ultimately flow out to Kvarnåkershamn on the western coast of Gotland. It might have been possible to travel by water from Bendesborg all the way to the Baltic Sea on Gotland's west side.
However, where the canal splits in Tänglings Myr, the other part of the canal flows down over Stånga, Lye, Lau (Lausmyr), and När, ultimately ending in Saigräum on the eastern coast of Gotland. This is detailed in Ormkvinnans barn (2006).
With more water on Gotland before the draining of wetlands, it was probably possible to travel across Gotland by boat via this water connection. And this may not have been the only route; it might also have been possible to travel from Västergarn and Bibos fishing village, across Roma to Åminne in Gothem on the eastern side of Gotland.