This is often called the Dative of the Person Judging (aka Dativus iudicantis; cf. also the "Dative of Relation": e.g. see this link), which is sometimes considered as a specific case of the "Dative of Reference" (see Sebastian's comment). An example of this kind of dative that is often found in many Latin textbooks is the following one:
oppidum primum Thessaliae venientibus ab Epiro (Caes. civ. 3. 80, 1).
The typical participle used in the dative case is replaced in your example from LLPSI by a dative pronoun (Iis) plus a relative subordinate clause (qui ad septentriones navigant). However, note that this is not surprising since both constructions can be claimed to function similarly. Cf. the example above from Caesar with "oppidum primum Thessaliae iis qui veniunt ab Epiro". Accordingly, a similar parallelism can be said to hold for your example:
Iis qui ad septentriones navigant... ≈ Navigantibus ad septentriones...
Since these datives are quite external to the main predication, there are some scholars who have even classified the dativus iudicantis as a "disjunct". For example, see Baños Baños's (2021: 225-226) section "El dativo como disjunto" ('The dative as a disjunct'), included in his Sintaxis Latina, vol. I (CSIC, Madrid). This Spanish author includes two different kinds of datives like the dativus iudicantis and "ethical datives" in a specific section on "disjunct datives", this inclusion being motivated by the fact that both datives are quite peripherical in the syntactic structure.
Finally, I think that it can be very useful for you, Charo, to take a look at this pdf document in Spanish, which provides a nice summary of the grammar of Latin cases with examples from Lingua latina per se illustrata. (NB: your dative is exemplified on page 5 in the subsection "[5.5] Dativo de relación").
