Assuming 4 cyberfins and two waterjets, IMHO:
+4 DP Swimming
+4 Physical Limit for Swimming Only
x3 Speed
so assuming a human with a swim skill of 6, an AGI6 & STR2 and a PLimit 5 with the above:
swims at 12meters per turn and sprints STr+Swimming[Limit 9], 3 meters per hit.
If an Elf or troll it would be 6 meters per hit.
If you think both speed multiples should apply, then
+4 DP Swimming
+4 Physical Limit for Swimming only
x5 speed
The aforementioned human would swim 20 meter/turn w/ 5 meters per hit (10 elf troll).
If it doesn't bother you that a troll with max AGI/STR could go 40 meters per turn with an average of 55 meters per turn sprinting while defaulting on the swim test, then apply both.
The first seems more logical to me, but players might think they were cheated if the full bonus of both items weren't applied. Logic should never stand in the way when making our characters.