Since I recommended this ink to someone who was unhappy with Nitrogen from Organics studio, I thought I would try to showcase it properly which is rather difficult for a sheening ink.

This is Ostrich's Sea of Okinawa from their sheen line, it presents as a bright blue with red sheen. They are available in 18mL plastic bottles, or 30mL glass bottles. The glass bottles are a lot more expensive per unit of volume (so I stick to the smaller plastic ones).

The Sea of Okinawa is a very well behaved sheen, and depending on the pen and paper it can even be rather subdued: broad wet nibs show it as a bright blue, as it does from this Picasso 902 M. But in this Pilot Custom 74 F, it shows darker, with no shading and a rather flat sheen. This ink is very well behaved on a lot of papers and barely impacts ink flow in the pens I have tried it in. It is a bit of a nib creeper.

This ink can show tremendous shading and sheen on the right paper and in the right pen, and in that respect, it is rather picky, as it favors broader nibs to show shading and variation in sheen. The Waterman Exception even shows a haloing effect with this ink and its rather broad M nib, that I have not seen with anything [and I stand corrected on that]. It behaves well enough to be used as a daily, in work/professional setting, no problem, as basic copy paper will not show outlandish sheen.

(Oxford Essentials, 90g/m² Optik paper)

On this Oxford "Optik paper+", the Picasso 902 M shows some interesting effects, some very light haloing, though not as much as the Waterman Exception M.

(Oxford Essentials notebook, 80g/m² Optik paper+)

Nib creep on Pilot Custom 74 F

Mild haloing on 80g/m² Optik paper+ from a Picasso 902 M

Sheen from a drop of ink on 90g/m² Optik paper, ink name written with steel nib dip pen

On some papers (Clairefontaine's velum papers for instance), it shows a bright red metallic sheen, on some others (like the Optik paper and the Optik paper+) it shows a little more subtle, but I have not seen it feather or bleed though any of the paper I use, including the super cheap Action notebook (still with decent 80g/m² paper). White paper obviously makes the sheen more noticeable, but it does work well with a broad variety of papers nonetheless, it will just be less noticeable. I have also used this ink in a Picasso 916 with a SEF nib that's thinner than Pilot's EF steel nibs, and it caused zero issues in terms of flow, and still showed sheen.

I apologize for not showing the ink in the Waterman Exception after bringing it up, but I just filled it with some Emeraude de Chivor and I love this ink way too much to waste even a drop of it. If anyone is really curious I can add a sample at a later date, but it's really hard to capture haloing.

Edit: I forgot to mention that Ostrich also has a more saturated version of it (that I have not tried) called Aegian sea. But after my experience with their Glen of the sea, an insanely complexe extremely dark purple that looks black, feathers pink and sheens bright metallic green on anything, including toilet paper, I have been reluctant to try it. Because the Glen of the sea has an extremely hard time flowing in a fountain pen, it crusts up everywhere, smudges (pink) even when dry. Diluting it doesn't really help much, it shows less sheen, but not more color and doesn't even improve the flow.