I'll assume you are running Navisphere Manager 26 (with FLARE 23) and a newer version of Java.  While running a recent version of Java is not an issue necessarily in itself; however, you most likely answered "Yes" to a Java prompt asking:

"Block potentially unsafe components from being run? (recommended)"

This appears when you access certain displays within Navisphere Manager such as "System Properties".  Instinctively, people select "Yes", and I would agree it is recommended on webpages you aren't familiar with; however, that will unfortunately break functionality within the GUI as you had experienced.  I will mention though if you managed your array with Unisphere (via domain management), you wouldn't get this prompt.

You can confirm you had (accidentally) answered: "Yes" if you only see the tabs:

1) General

2) Cache  (NOTE: even though the Cache tab is displayed, several background Java scripts don't run)

On the other hand if you properly selected "No", then you'd see many more tabs:

1) General

2) Cache

3) Memory

4) Storage Access

5) Software

You have 2 solutions (*neither* requires disabling write cache or watermarks):

Solution #1

1) Close the GUI and clear both browser and java cache

2) When prompted, select "No" to the Java prompt

3) On the Cache tab modify in the following order:

a) Change the High Watermark first

NOTE: This is important as the GUI checks that Low watermark is < High watermark.  If you were to try to change the Low watermark first, it would appear to not accept your input.  However, in this case the new lw (60%) > old hw (50%).

b) Change the Low Watermark, then click OK

Solution #2

Modify via CLI:

1) naviseccli -h setcache -l 60 -h 80

NOTE: only need to run on one SP as it is a "mirrored" option

2) naviseccli -h getcache

3) naviseccli -h getcache

NOTE: (optional) verify that both SP's report the updated values