You have two questions there. I will try to answer both:
1) Triangulation - if you can reasonably accurately pace out a 100 metre baseline and can estimate angles to within a few degrees then a rough estimate of distance is possible.
An example from eso.org - in reality it is much easier than this as you can use very rough estimations to make the calculations easier:
Measure of distance by triangulation
We wish to measure the distance CH between a building C and a road ABH that lies along the north-south direction in which an observer is travelling. The observer can measure only angles or distances along the road. From the position A, the observer measures an angle of azimuth 30° between the building C and the South direction;
from a position B located one kilometre further along the road, the observer measures an azimuth 45°. To calculate CH, it is sufficient to solve the triangle ABC in order to calculate the CH distance knowing the two angles in A and B and the length of the baseline AB. We apply one of the relationships between angles and sides in a triangle.
a/sin(A) = B/sin(B) = C/sin(C), where (A), (B), (C) are the angles A, B, C of the triangle ABC.
So we have: a/sin(30°) = 1 km/sin(C) where (C) = 180° - ((A)+(B)) = 180° -165° = 15°, and therefore
a = sin(30°)/sin(15°.
But CH = a.sin(B) = sin(45°), and therefore
CH = 1366 m
2) How good a swimmer are you, how cold is the water, and how strong is the tide?
In cold water even a strong swimmer can find it difficult to make any distance.
Try measuring the tide by throwing a small object out into the water and watching it over 10 minutes.
In waters around the UK, swimming a kilometre is probably not an issue for a strong swimmer - if the water is calm and the tide is slack. Before setting off try and line up two objects on the far shore so you can assess how much the tide is pushing you off course.