According to the datasheet, the “RS232” models MB736x are 0-Vcc (typical RS232 is -13V to +13V). Of course, it will be inverted from TTL, but if you account for that in your softwareSerial constructor, I do not see why you should need an external converter (MAX232 or similar). In fact, the converter might cause problems if it’s shifting the logic levels on the sensor side. Disclaimer: I haven’t done this myself; I’ve only used the true-TTL MB7389 and the I2C versions. Feel free to correct me if you have experience with it.
On the subject of sensor selection for future reference, the 7369/7389 might be preferable for water surface detection because they report the “largest” return rather than the “nearest” return. I can attest that the MB7389 does a good job of ignoring small objects partially obstructing its field of view. If the sensor field of view is relatively clear it’s probably a non-issue.