Sur mer is a virtuosic yet delicate etude that mimics the undulating motion of the sea. It is a prime example of late-Romantic Russian piano writing, requiring refined control of texture and color.
The primary challenge is maintaining the constant undulating 16th-note figuration while highlighting the hidden melodic contours.
Comparable difficulty and character — good candidates for what to learn next.
'Sur mer' (Etude) (Op. 14) is rated Henle level 7 of 9 — late advanced repertoire, roughly ABRSM grade 8 and above. In the RCM system it corresponds to about level 9.
You should be comfortable at Henle level 7 — meaning you can already play level 6–7 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 'Sur mer' (Etude) (Op. 14) lasts about 4 minutes.
'Sur mer' (Etude) (Op. 14) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Ballade in the Form of Variations by Felix Blumenfeld, Allegro de concert by Felix Blumenfeld, Barcarole by Joseph Marx — see the full similar-pieces list on this page.